The Crimson Binome
Hero
I'm not convinced that clothies with no Con bonus are a thing that exist in 5E. In the last game I played, the average Con score at the end of the campaign was around 18. As long as everyone focused on offense, you really could just charge into the fray and kill them before they killed you, and you'd always come out on top. Due to the action economy, no BBEG is really capable of dealing damage quickly enough to win that exchange.5E every much still got Squishies; there's a huge life expectancy discrepancy between a clothie with middling AC and no Con bonus, and someone with 20+ AC and twice as many hp.
In particular, 5E doesn't allow for stratospheric AC; you can get reliable AC against minions, but bosses will always have at least a 30% chance to hit you, so you can't just stand there forever. The diminishing return is on the effort; you have to work much harder to get your AC from 26 to 27, than you do to get it from 23 to 24, and it doesn't help you at all against anything that would already need a 20 to hit you. Pathfinder 1E is much the same, but increase the AC numbers by +10, and the boss's attack bonus by +20. In Pathfinder, if you don't throw absolutely everything you have into AC, such that normal minions need a natural 20 to hit, then the high-level boss monsters will only miss you on a 1.Remember, we're discussing this in the context of "too high" AC. You want characters that focus on defense to get great AC but not stratospheric AC.
This probably needs a diminishing returns mechanism. Each extra bonus yields less and less. Just adding +1 after +1 (and certainly not +2s) simply won't work.
On a side note, because AC in these games works as avoidance rather than mitigation, having a high AC can significantly reduce your ability to redirect attacks away from your allies. The enemy isn't going to keep swinging at you, if they roll an 18 and miss. It's not really a huge deal in either game, since enemies who miss on an 18 are also dead by round 3, but it's kind of annoying.