CapnZapp
Legend
Any build doing few big attacks won't and shouldn't use GWM/SS, no.You're right that I did part of that math wrong. If your base is +6/8, you get +1/18 as an option. Then if you can "mitigate", you now have +6/18 or +11/8. And +11/8 is not generally as good damage-wise as +6/18.
I do think it's worth looking at other things that affect damage. If you've got a rogue getting sneak attack, for instance, +6d6 damage on a hit hugely alters outcomes; that's +21 damage. If we replace 8/18 with 29/39 damage, +11 at 29 average damage is better than +6 at 39 average damage for anything AC 13 or higher; basically, the only point at which the to-hit penalty comes out ahead is when part of the to-hit bonus is useless because one always misses. Of course, rogue isn't getting Extra Attack.
So some of the benefit of the -5/+10 is specific to being able to get lots of attacks which are otherwise relatively low damage. The higher the base damage is, the less significant the +10 is.
And since bounded accuracy means that Mooks Are Viable, it's pretty easy to just toss people a field full of 8hp targets, where doing 20+ damage to them is a waste of effort.
Not coincidentally I'd far prefer if Sneak Attack actually mean doing lots of damage compared to fighters. I mean, they're far more frail. To me a Rogue works best as a glass cannon archetype, especially since the skill monkey niche has shrunk so much in this edition.
In fact I think Sneak damage should probably be even higher, even if we remove GWM/SS from fighters. My players are considering a few Rogue levels, but only as complement to a fighter chassi. One d6 per two levels is actually not that compelling since its only once/turn. But that's a different discussion

8 hp Mooks are trivially wiped away by other tools than weapons. At least in any combat that's actually challenging. That's the reason I'm not overly concerned about "overdamage" in my calculations. (I'm firmly convinced lots of people make the mistake of including "easy" combats into their DPR calculations. But it matters much more how effective you are when the stakes are high than when they're not! DPR math might show you how ineffective overdamage can be, but in real play there are only two scenarios: either the combat is actually challenging, and then the mooks will be dealt with by somebody else than the fighter, possibly using limited resources such as area spells (which is okay since there won't be many such fights this day); or the combat isn't challenging, and then nobody cares that the fighter might be losing some DPR). Thus I posit that overdamage isn't a real issue - it's more of a DPR construct than something that you ought to take into account when building your character.