The situation mentioned advantage.3 you seem to be forgetting to apply that -5 to hit in your math. That's a pretty important factor.
At most levels with advantage, -5 doesn't hurt that much.
At low levels vs higher AC targets, ya.
Assume level 4 and 16 stat for +6 to hit.
Like 15 AC at +1 to hit is about 55% hit chance, vs 84% without -5. 84/55 is +50% dpr.
Hit level 8 with 18 str and a +1 weapon and it is +4/+9 or 75% vs 88%, a mere 7/8/(3/4) or 17% dpr increase.
And that is ignoring crits (which help the gwm).