My analysis is specific to GWM, so your sharpshooter/archery style example is not really relevant - I am not suggesting SS is not OP, just GWM.
As for GWM, I already addressed your point when I said "In order for it to be a factor, you would have to have so many buffs that your target to hit # goes below 5 WHEN GWM is on, something that shouldn't really ever happen in a competitive game." Your level 8 guy w/GWM would likely only have +8 to hit. But the typical AC he should be facing at that level is 16 as per DMG. So unless he gets more than +6 in buffs, or fights a lot of AC9 or less monsters, or some combination thereof, he's not gonna hit the floor except rarely. Is it possible? Sure - but if it happens more than rarely your DM should be giving you better challenges lol....
I, personally, think it's very odd if the average AC you're facing at level 8 is AC 16. That puts optimized characters at a 50% hit rate, and that's not what the game's designed for. Also, that would mean that you're doing a lot of fighting single monsters as a CR 8 against a party of 4 8th level characters is a medium challenge. 2 are a deadly one. 4 CR 2 critters is also a medium challenge, and have ACs much lower than 16. I don't think your argument that 8th level means average AC 16 enemies holds much water.
But, yes, the floors are lower for non-archery style characters. That's a true statement. I went with SS because I have a character in my game that fit that bill precisely, and I knew the point stood and came up often enough to matter -- that the penalty was moot for low AC (ie, 12 and lower) critters.
I read the post. It's always a factor as long as you don't hit on a -3 normally or only on a natural 20, otherwise it affects your accuracy. The only areas which the -5 isn't a full -5 is when you chance of hitting can't improve or worsen by a full 5. We're talking miss on a 15 or hit on a 1 (if it weren't a fumble). That's the rare event.
Not if you add in Bless, or a Bard. Or advantage. Point is, that 2+attack bonus is the floor against which you gain no benefit from bonuses (you gain a slight one from advantage). So long as the enemies are close to 2+attack bonus, using the -5/+10 gains full use from any buffs including advantage. If the AC you're aiming at is higher than 2+attack bonus, the advantage drops off -- for instance, at 3+attack bonus, you only ever get a +1 benefit from any bonus die or effect, but the -5 guy is still getting all of it. If you both roll 4 on the Bless die, your chance to hit increases by 5%, their's increases by 20%.
Depending on the AC and the bonus dice, you can completely erase the -5 penalty without ever giving any advantage to a non-SS/GWM character. So, the point that you can erase the penalty exists, and it's not much of a corner case. Bard's get d6's right off the bat.