Why?
Off the top of my head, here's four things you shouldn't try Sharpshooter against:
(1) Spellcasting dragons w/ Shield spell (AC 24)
(2) Drow Elite Warriors shooting at you from beyond your darkvision range (AC 18 + disadvantage)
(3) Nycaloths inside of Darkness spells (AC 18 + disadvantage)
(4) Almost anything that isn't a mook while you're being constricted by a giant snake/toad/octopus or caught in a Web spell or whatever (typically AC 15-18 + disadvantage)
The decision point for Sharpshooter doesn't always come into play, but sometimes it does (GWM even moreso), and PCs now have an additional reason to jockey for advantage over disadvantage. And that adds interest to the fight (at least for me as a DM!).
Forcing players to make decisions is what make creating scenarios and running scenarios interesting. Teaching them to "solve" the puzzle of how to deal with drow in the dark is fun.
Sure, some things have AC over 20. That's why I used that key word, "essentially." There are, in fact, 11 monsters in the Monster Manual with AC 21 or higher (Tarrasque, Empyrian, Solar, 4 ancient metallics, and 4 ancient chromatics). You probably
don't want to use power attack against those until you're at very high level. However, you're
not going to spend levels 10 to 20 fighting nothing but those creatures.
Well, how about spellcasters? If we exclude monsters with base AC below 16 (since 15 + 5 = 20) and also exclude those counted above, there are... 33 spellcasting stat blocks in the Monster Manual: Androsphinx, Arcanaloth, Cambion, Couatl, Dao, Death Knight, Death Slaad, Deva, Djinni, Drider, Efreeti, Githyanki Knight, Githyanki Warrior, Githzerai Zerth, Glabrezu, Gray Slaad, Green Hag, Green Slaad, Guardian Naga, Gynosphinx, Lich, Marid, Mezzoloth, Mummy Lord, Night Hag, Nycaloth, Oni, Orc Eye of Gruumsh, Pit Fiend, Planetar, Rakshasa, Storm Giant, Ultroloth. [For my source, Google "5e monster manual spreadsheet" and you'll likely find what I did.] Now, a lot of things on that list are likely to be friendly NPCs (one side or the other). And not everything else is actually a real spellcaster. A lot of them just have "Innate Spellcasting" instead of "Spellcaster" and just have a handful of spells that they get each day (Storm Giants, Genies, Githyanki, most fiends, etc.). There's no real chance that an Efreeti is casting
darkness or
shield, right? So, some of them might get
shield,
darkness, or some other magical effect that can raise AC... but most do not. And even then... for every
darkness there is a
daylight or
dispel or Reckless Attack. For every
shield there is a
bless or Precision Attack.
And yeah, congratulations, you as a DM can construct encounters where typically good choices are instead bad. Are you saying you're planning to do that
every encounter? I mean, that's pretty BS if you do, isn't it? You're not fixing the problem. You're just mitigating it without telling the players you're going to do that, preventing them from making an informed choice about the game. That's pretty poor style, and is going to upset your players. "Here's an optimal choice, but it will
never be relevant because I'm never going allow it to be." I mean, that clearly can't be what you mean, right? No one would expect players to stick around for a game that pulled something like that? That's like giving out only magical swords, and then making every enemy resistant or immune to magical slashing damage. Nobody is really going to do that
every encounter.
So, no, I really don't think the existence of mitigating circumstances changes anything. They're going to be uncommon enough that it isn't likely to come up all day every encounter. It doesn't change the fact that once you get to mid level your default mode of attack in any given encounter should be to use power attack. All things being equal, power attack is the superior default choice. Just like using a magical sword over a mundane mace is the superior default choice even though you might encounter skeletons or oozes.