All of this is misdirection. A reasonable person doesn't bother setting a DC for jumping to the moon or surfing on clouds.
What's being done has to still be in the realm of possibility.
Yes, exactly! Some things are impossible, and therefore by definition shouldn't be possible 5% of the time.
I agree that players shouldn't even have to roll for impossible tasks (except maybe to determine the degree of failure), but Larian doesn't know what every character's bonus will be. So if they have a DC 35 (or 25, or 30) check, they need to let you roll even if your bonus is +0. But such a character shouldn't succeed on a 20; that just cheapens the effort another player spent getting their bonus to +16
A natural 20 on an attack roll represents the best hit a character can do, which is a lucky or skilled hard hit. They've bypassed the defenses of a fallible creature. Because HP is abstracted in 5e, you could even say that such a lucky hit is actually only doing stamina or armor damage.
So make the AC 70. Or 1,000. It's the same argument.
It's not the same argument because AC is actually representing a contested check, but is simplified so that the players & the DM don't have to "roll for defense." The enemy is actively trying to not be hit, whereas a tightrope isn't actively trying to shake someone off.
AC is effectively taking 10 on a check: 10 + armor + dex. Instead (and as some other rpgs do), the attacker could roll 1d20+attack bonus, and the defender could roll 1d20+AC bonus. The "natural 20=auto hit" rule simulates the defender rolling low on their d20.
E.g., An enemy has AC 30 and you have a to-hit bonus of 9. Without the auto-success-on-natural-20 rule, that player couldn't hit the enemy.
But in reality the enemy is trying to not be hit, and could mess up. We swap over to the 'contested attack' system, and the enemy rolls a 2 (so AC check of 2+20=22) and the player rolls a 19 (to-hit- of 19+9=28), which means they've hit!
And because of Bounded Accuracy, pretty much all ACs will be between 7 and 30, so we don't worry about edge cases like someone having an AC of 50. Practically by definition, it's impossible for someone to have that large of an AC in 5e. But it's *not* impossible for an ability check to be that high, because, as you say, some things aren't in the realm of possibility.
