A weird thing about monsters

[KorvinStarmast]

Does this match up with what people have observed using these systems?

No. As you go higher in level, your TEAM also has resources to mitigate save short comings. Things like Bardic inspiration, paladin auras, spells like Heroes Feast or Freedom of Movement, and so on. (Monk's save boost has already been mentioned). For possession, prot from evil and good at level 1 is kinda nice.

With Tasha's coming out, though, a whole bunch of saves show up against INT; who has INT save proficiency? Wizards and Rogues IIRC druids. That changed the landscape a little bit. I found the "you have six saves and you are only proficiency in two" structure to be bad when I began the edition. I have gotten used to it but asm still not a huge fan. The original game had all saves become more makeable as you increased in level, and Spell Save DC wasn't a thing until WotC showed up.
I am not sure I would mind boing back to Reflex, Fortitude, and Will just for the KISS principle.

Saves don't really work like this because DCs scale beyond what most characters can reasonably resist.

Yes. Higher level play needs to be more dangerous, and we have found that in Tier 3 and Tier 4 it often is depending on what we are up against. Going up against casters at a high level can be brutal. That's not a bad thing. Why should high level play be a cake walk?
Monsters Higher than CR 22 are not very common in the MM, which is what the game is based on. Some of the later stuff (MToF) is the usual power creep at work ...

I think the quote was something like, 'success and failure should be determined by the player's cleverness, not what's on their character sheet'

Try using teamwork. The basic unit of D&D is The Party. The YHoutubers are treating the PC as a single player avatar in a video game.

I think that saves do kinda fail at this

Given that the devs were talking about Ability Checks, you tossed out a non sequitur.

3.5 character gets decked out in expected magic gear but the poor 5e guy can't even scrounge up a Cloak of Protection :(

My players found one of those in a Tier 1 adventure, Ghosts of Saltmarsh. The devs drop plenty of goodies in the published adventures, so I am guessing that the intent is for magic items to be in the game. Just what magic items will vary.

Blame the devs for saying "if you don't include magic items in your game, that's fine, it's not gonna break". Which is a lie if your game continues after level 5 to 6 or so.

Not sure if you paraphrased that correctly, they were signaling the end of WBL forced power creep. One of the nice things about Xanathar's was the explanation of the hundred items, and distribution, that was expected to drop in a 1-20 adventure for a four person party.

But even a Cloak of Protection, like, +2 (which does not exist RAW

Indeed. you cannot make a PC invulnerable. My Champion just hit level 17. He has a RoP, Cloak of Prot, +1 Shield, +1 Half Plate, Dex of 16 and Medium armor master. His AC is 25. He still gets hit. We work as a team to slow the enemy down, break up groups, or inflict debuffs to mitigate damage; you can't eliminate it.

I don't mind people (especially me) having weaknesses but saves tend to outright remove a character from participation.

Yes. There are some saves (like Finger of Death) that kill the PC. But at Tiers 3 and4 you can expect a Raise Dead or similar spell to exist. If the PARTY works together, a lot of "death" becomes a short term thing. Granted, Disintegrate can end the character.

The highest claim here is banshee and demilich, with both have reduce hp to 0 attacks, saves are the only defense available.

Yes. Messing with a demilich can get you killed. (Banshee at low level can be downright scary. The party I run as DM in my brother's world ran into one in an old abandoned tower and a few of them went into death saves based on her ability. It was a nasty encounter, to be sure).