Well yeah of course the math looks bad if you give the target a huge AC and ignore Advantage. If you built a Level 20 character without guaranteed Advantage on demand then you are doing it very wrong, with or without GWM. Even against 22 AC, that should be 16 damage per attack with GWM and 13.2 without GWM
Is AC 22 a huge AC for a significant encounter at level 20? I picked a dragon because I felt it rather typical; if I wanted to sandbag I could have gone much higher.
A level 20 character with guaranteed advantage on demand? So, a rogue or barbarian (which has a significant opportunity cost)? Or just a really accommodating DM? But even with advantage on demand, you only get a slight DPR increase (my math says 17 vs. 14.1), and that's only by ignoring the possibility of additional damage increasers that get nerfed by GWM. I notice you don't address what happens if, say, that attacker also has holy weapon cast on them; that's probably because GWM then remains a DPR decrease even
with advantage on demand.
The reason for that is, as I noted, that the penalty for GWM is multiplicative, meaning that it becomes stronger as your damage gets higher, at some point inevitably rendering the feat a damage decrease, since the bonus is a static +10. There are a lot of factors that affect where that line will be drawn in a particular encounter, and AC is a big one, but the math on GWM means that the more damage you do, the less effective it becomes, regardless of AC. In other words, it is designed so that it scales poorly.
It is a situational feat at best, mostly useful for generating big hits against soft opponents.
I completely missed this! That is a huge deal, I'm over here comparing a half-feat to a full-feat like they ought to be the same damage output
Anyone optimizing properly will already have advantage on demand - all you're really doing with Topple (at least damage-wise) is screwing over your ranged attackers who now have to straight roll
Obviously knocking someone probe has to take into account party composition and the situation, but I think most will agree that it, and most of the other weapon mastery traits, are extremely potent new tools to have in the kit, that will typically be used in ways that are tactically important and usually lead to significant DPR gains. That is certainly what play testing is showing us.