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.
This.
Or rather, obviously nobody expects you to have infinite staying power. What would be nice, and what would qualify your build as a "tank", would be to be able to reliably keep standing for the three first few rounds, with a shot as bonus rounds depending on die rolls.
That is, enough to actually tank a significant foe. (Combats are indeed short, so the difference between a promising tank build and a disappointing tank build can be as little as a few rounds worth of staying power).
Which brings us to the (Bear) Barbarian: effectively having twice the hp of anyone else is the only real tanking ability that stands out from the crowd.
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.
My experience is the reverse. Increasing your AC from, say, 20 to 21 doesn't do you much good, when viewed from the "Am I a tank" perspective.
Sure the incoming DPS is lessened. But the amount of avoidance (from high-threat foes) is not meaningfully reduced, i.e. you're no more a tank just because you effectively have 5% more hp. Not when TWICE the hp is the kind of change you want - at minimum.
What makes a player consider his or her build "successful" is much less some kind of average effectiveness. Much more important is to succeed when it counts. Nobody boasts about taking 7,3% less damage from some trivial goblin encounter. Everybody boasts about enduring the crushing blows from the giant or dragon, while the rest of the time brings it low.
Once your AC reaches 24, 25, 26... something finally changes significantly. Now only BBEGs hit you on anything less than a 20. Find a meaningful dose of temp hp and (ideally) crit immunity and you can finally call yourself a proper tank (assuming you're at a healthy hp total otherwise).
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.
This only reinforces my belief that lower numbers (what's sometimes called "bounded accuracy") makes it easier to design a game where you retain the sweetspot longer and easier. That is, a game where you can reach 30% or lower hit rate, but not lower than 10%.
What I meant was this: if each bonus lowers the hit ratio by 5pp (percentage points), once you reach 30%, maybe each bonus only lowers the hit ratio by 2pp, and once you reach 15%, only by 1pp. (Not saying this is practical for a simple game).
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.
Agreed.
Just because WotC avoids "aggro mechanics" like the plague doesn't mean PF2 has to.