how would monsters adapt in a high power society?

[King of Nowhere]

stuff

you raise many good points. I can answer some of those, because I already have a pretty good framework: I DMed a campaign in this world, now it's the same world about one century later. the previous campaign already had several elements of progress over a traditional fantasy world, now i'm further exploring those elements as the tech moves further.
"traditional" pollution and land overuse is, luckily, something that did not happen, because of druids and feys. those were able to put the concept of "sustainable development" into the general culture earlier than it did in the real world. nymphs are especially good at persuading the general public that their wilderness homes should be preserved. traditional monsters would keep exhisting in those wilderness.
magic pollution instead is going to be a thing. I already established from the previous campaign that high concentrations of magic can have dangerous side effects. also, magic is not an infinite resource; overexploitation of the thaumic field can be a problem.

blood sports with monsters already exhisted in the previous campaign: "dungeoning" is an extreme sport where groups of adventurers are sent in an artificial dungeoning against monsters for the entertainment of a paying public. I have it as an excuse to pit the party against dumb brutes (something that already didn't make much sense in the previous campaign)

Problem is, this won't create traditional "monster" problems to be solved by adventurers. It will be a problem for animal control, or exterminators. And the most likely danger from pests monsters will be indirect: they damage physical or magical infrastructure, causing something to fail, causing injury and death.

low level parties can be just that: pest control. In fact, having established that magic can mutate normal creatures into random monsters, it means that those pests cannot ever be exterminated completely. given the high amount of magic around, there will be even more of those monsters. this can be a good low level plot.
those monsters will rarely pose a direct death threat, but they may damage stuff. As in my idea of the swarm that drains your magic items.

Magical infrastructure...magical education and golems and adventurer wizard expert terms and items...is no more likely to be distributed according to need than, say...real life distribution of food and shelter.

that was already a worldbuilding point of the previous campaign. I established in the "lived happily ever after" end of the previous campaign that the situation improved everywhere, and I have a hard time imagining that 100 years later there would still be places where it would be normal to get eaten by a random monster. just like even in the poorest places on earth it is not commonplace for people to be eaten by lions.
those places have other problems, though. crime - the kind that is low-key enough to not send in high level people to deal with it - can still be an adventure hook. paladins generally volunteer to protect those places.

The folly of literary magic-users is like speed run of the folly that overtakes real people who just never have to hear the word "no." In a full magic society an archmage is still a WMD...how do you call them to account, if they decide to breach laws and norms?

there were still enough high level resources to deal with a rogue murderhobo in the previous campaign. an archmage kidnapping people en masse for experiment would be taken down fast enough.
on the other hand, an archmage that was sponsored and protected by a powerful nation, or other powerful organization, could enjoy much less accountability. some of the evil nations were basically kept together by those powerful evil people who had to bunch together to protect themselves from high level heroes. some of the most blatant cases were dealt with in the previous campaign, but that is by no means an obstacle to recreating the same dynamics.

"Menial jobs will be done by undead and golems" kind of glides over the question of whose corpses are doing what, potentially forever. So on the low end of complexity, mechanized undeath means there's either a market for dead bodies, or a legal and social norms in which the dead are turned over to converted into mechanized undead as a civic duty.

the previous campaign already had this in a less widespread form. there's basically a market: you can sign up a paper, when you die your body is turned into an undead and a compensation will be paid to your descendants. Less scrupolous powers would do that to all their citizens.
what happens when the demand cannot be satisfied by the natural death rate? potential plot hooks...

But more sophisticated forms of undead are available--vampirism, lichdom--where personality and skills are retained, and...theoretically...there's going to be both practical and perverse incentives to reach for these. Civic-minded undeath might include weighing the option to take on a role forever by intentionally assuming sapient undeath; powerful individuals might decide to transform themselves so as to never have to release their assumed titles and authority.

there's even a guy who keeps working from the afterlife. A skilled administrator whose status can be roughly compared discworld's vetinari, he did not try to go undead because it was too risky, but his underlings keep planeshifting into the afterlife to ask him directions . nobody tried to take his place because events from the previous campaign made clear that nobody has the skill to handle it.

However...what if you had a great deal of power, an ability to duck accountability, and wanted one of your employees to not leave (not die), to keep doing their research job or their particular skilled craft? Can you make an undead that retains its skill with a lathe, to do nothing but turn out coffee table legs? Can you strip out the bits of free will that make people complain and job hunt, but keep the bits that make an excellent lab worker? Even if there were rules against doing these kinds of things, the incentive to do it would always be there.

wouldn't that be a bit expensive to get just one worker? especially because you already have cheap automated workers. you'd have to do that to people with an excellence job to make it worthwhile, and to keep them good at their job you'd have to leave a good chunk of their intelligence... i wonder what could possibly go wrong?
could be used as plot point for the rebellion of the ghost office workers.

Relative to the described present, there's going to be a moment in the past where the clever monster types--stuff like rakshasa and fey and dragons--notice the shift in human(oid) societal structure and start to alter their own behavioral patterns. Some will choose direct resistance, some will choose flight...but the interesting possibility is those that choose collaboration.

already started. the previous campaing ended with a dragon elder trying to organize a movement to fight the humanoids... and getting trounced. hard.
the remaining dragons choose cooperation. many of them got jobs in human societies, and the same goes for many other magical creatures.
dragon pride still runs strong, though. while none of them believes anymore that they should be revered as gods and offered sacrifices, many still think that they should be A-class citizens because of their greater abilities. And some did not like how six humanoids and one golem defeated the three most powerful of their kind, and will want a rematch.

To get specific:

The first is that "natural" undeath is a kind of trace of the living person: ghosts with "unfinished business," or trapped repeating some loop of traumatic experience and emotion; gestalt undead that represent many people. More people, more concentrated, continuously surrounded by magic is likely to produce unusual new permutations of undeath that occur naturally. Giant urban gestalts fed by the ennui and resentment of people that lived and died as middle managers; drudgery ghosts continuing to go through the motions of their job in shutdown factories full of broken machinery; office wights bound to their desk.

Hive mind rats fit into any space available, can eat anything, but as the colony grow it becomes more intelligent. Most of the time this just means they're smart enough to stay undetected, eat surreptitiously, and hide signs of themselves. But sometimes a colony is both large enough and threatened enough to become proactively aggressive and develop elaborate plans for securing a home. Such colonies have committed murders, attempted takeovers of whole buildings, and have tried to cobble together tools and magical items to create elaborate traps and weapons to deter humans.

Conductor slimes' primary behavior is to attached to power lines and feed off voltage and latent magical energies, but they go undetected because they stretch to cover as much cable as possible--so thin as to be barely discernible to the eye. The problem arises when a slime infestation remains undetected, and individuals become excessively large or the population becomes overnumerous and they seek out other food sources in a panic.

Shadow anglers: ambush predators that can easily slip back and forth across planar barriers that create a "lure"--often in the form of a person-like shape--that beckons, gestures, or otherwise draws in people before the creature bounds across the planar boundary to attack and consume their target. They always existed, but their specific skill make them comparatively successful at hunting in an age with a great deal of prophylactic magic. The ones that target less noticeable targets, like pets, tend to be more successful, but as they grow in size they invariably switch to humans.

Portal mimics are also ambush attackers, the creatures have learned to shapeshift into a variety of forms resembling everyday items. The only consistent feature being the the mouth of the mimics corresponds to a door or other opening that a person would reach out and touch. By waiting until the exact moment of contact--a hand reaches for a doorknob, a foot lands on a manhole--they manage to pounce, grapple, and savage prey to death quickly. Rumor has it that there is a more sophisticated variety taking on the form of taxis and other kinds of public transport, but they are unverified.

Gremlins are fey that manifest in places where large pieces of machinery are concentrated. Some are merely mischevious, but some are homicidal malicious, specifically using their ability to control machines to create accidents and harm. Knockers are fey originally connected to mines, but now associated with all kinds of complex engineered structures...and they have no mischief in them, they devote all their energy and power to destroying and collapsing buildings. Poliads are generally benigh spirits who existence are connected to large buildings (generally residential buildings and offices rather than factories), but can become very dangerous if their territory is damaged or degraded. There are other, less understood spirits: the ones that live on the roadways as hitchhikers bringing little blessings and curses: the ones that build castles in the shadows of the subway tunnels, coaxing passengers to step in front of oncoming cars and become their permanent guests; the voices that whisper from electrical outlets and the figures that follow, peering at you, from billets and posters, block after block.

Contraelementals are a rare and disruptive spirit phenomenon that occur in locations with dense technology and magic overlaid. In their presence, complex objects begin to unmake, collapsing into base elements that re-compose as swarms of varying-size elementals. Items with magic charges do not simply become inert, but discharge their power as kind of expanding anti-magic field. When it first manifests, a contraelementals is only perceptible with detection magics, and resembles an expanding and contracted spiral of energy. As it grows, though it takes on both a rough humanoid shape and displays agency, moving toward and targeting objects and locations for unmaking.

A brazen head is a colloquial term for the core magic and instructions that animate a golem, but is also used to describe a particular kind of industrial accident inherent to golem assembly and programming: when the core animating instructions activate abnormally, the semi-consciousness of the golem extending to include other objects in a variable radius. In attempting to be a golem, a brazen head animates and draws together everything around it--included other programmed homonculi and people--creating a rough, chaotic, but recognizably humanoid form with whatever is available, then begins to execute a degraded version of its core functionality. In a society where entire industries operate on golem labor and golems are heavily incorporated into security, this kind of malfunction can cross from an containable hazard to a kind of magical meltdown in which entire factories and industrial zones collapses together to form a rampaging colossus.

The sheer profusion of magic in densely-populated areas leads to a kind of pollution problem. Living spells feed off the magic power of individuals and objects. Magic missiles swarm like flies. The football-sized kernel of a fireball rolls through the air, expanding like a puffer fish to defend itself. Transmogrified animals are just mundane pests that can cast a spell at will. Miasmas are magical smog with chaotic effects on those caught inside...but they seem to seek out targets rather than just settle at low points.

those are all good ideas. Thanks!