Where are you getting these absolute numbers from? I never said whether it was 3 Stygian Skeletons or 11. It could be the difference between 6 hits and 4 hits, or 60 hits and 40 hits.
No, you said 20 attacks. That's why I said "your example". But it carries throughout. You're going to go from 4 to 3 hits from the goblin, 6 to 5 hits from the minotaur, 10 to 9 hits from the dragon, whatever the magic # of hits per 20 attacks is per monster. 1 harder to hit is pretty useful no matter what, and is good at all levels in this system. Your method leads to it seeming to be disproportionately useful to the super-defensive character, when in actuality the group takes the same amount of damage whoever takes it as long as that character gets attacked as much.
In 3e/PF, it can be a lot more complicated, where things need 2s to hit, but maybe not for iterative attacks, or maybe it affects the amount they power attack.
And computing relative damage reduction is easy!
Okay, let's take a level 10 fighter with +1 plate, deciding between the different fighting styles. Assume that he'll fight a reasonable number of every foe up to CR 15, some of which will attack AC, some of which will call for saves, some of which will get advantage or not as appropriate, and some of which will just ignore the fighter. How much does +1 AC impact the damage he takes, for all those monsters, in some reasonable metric? How does it compare to how much less damage he'd take if he killed things faster? Did you take into account DM habits for focusing or spreading fire, or metagaming?
Cause, yeah, boiling it down to "If the enemy needs a 4 to hit, +1 makes that a 3 so they do 25% less damage" is certainly a statement you can make, much like "If the dragon needs a 10 to hit, +1 makes that a 9 so you take 10% less damage from its melee attacks. When it does those, instead of its wing claps and breath weapons." One is maybe more optimistic than the other.
It's not +1 AC is enemies hit you 5% less of the amount they already hit you. It's a 5% off the percentage they already did. So yeah, 20% goes to 15%, 50% goes to 45%. I think most people can cope with that, rather than "Well, he was hitting me for 20 damage a round so now he'll hit me for 19 damage a round?" - the people who do that level of math understand enough to follow through.
Your argument boils down to "computing advantage and disadvantage is too hard."
Well, it is, since you don't know whether you'll have them. Unless you're a halfling rogue/warlock with devil's sight in darkness in which case you can probably handwave it.