Why play 3.5 anymore?

[Morcleon]

Is "bounded accuracy" ever actually given a hard mechanical guideline, or is it just an aspiration to which, so far, the designers have mostly adhered? Because nothing I have seen in the PHB suggests that it's impossible for monsters to go up to 30+ AC, nor for bonuses not to be found to stack similarly high to attack. They just don't, currently.

Yes, it'd be power creep, but it would not be, unless I am missing something, actually breaking the rules of the game.

It's not explicitly stated, but is made that way by capping PC stats at 20, making AC incredibly hard to increase, as well as removing basically all permanent bonuses to things outside of the normal formula.