It's not broken in our games so we don't need to fix it.
If you don't mind that it's broken (or, for that matter, revel in it), you don't need to fix it in your games.
GWM, SS, CE and PM are all designed to be above the power curve while also being completely unnecessary to take to play the game. It is an intentional design decision by WotC.
Interesting as the overkill/'trap' insight is, I doubt it's a design intention - but we can't know unless they come out and admit it (the way Monty Cook came out and admitted that 3e was designed to reward system mastery, for instance, or the way Mike has done with regards to the Firball spell).

My guess is that the feats are there to feel more like 3.x (which, y'know, rewarded system mastery, so, yeah, maybe they're less balanced and some are ahead of the curve - but not as 'traps,' as callbacks to 3.x feel), but that's all it is, a guess.
I am going to take that smiley as indication you're not using that story to dismiss the concerns over GWM.
Of course not, it's not proof there's no issue, nor a solution, per se. Rather, it's a legitimate style or attitude under which the issue isn't too important - there might be conscious 'player restraint' involved (The two best & longest 3e campaigns I've been in featured a lot of that), or it might flow naturally from the table's prefernces.
The issue of softness has no bearing upon the discussion about -5/+10.
Sure it does - the experience of the game as lacking challenge is.a manifestation of such imbalances aligning 'high' across the group (ironically making the intra-party dynamic 'balanced').
This mechanism is severely broken regardless of whether you feel the game is challenging enough or not.
It may be OP compared to taking an ASI or a lot of other feats, it may be crucial balance in another sense, as a component of a build that lets a class be competitive when it otherwise might not be. So maybe just plain broken, maybe intentionally so, to balance with other brokenness, or maybe intentionally so to feel enough like power builds in past editions? :shrug: Like all feats, though, it's optional - and opting into feats makes 5e more like 3e - with more customization options, and greater rewards for system mastery.
The issue is that fighters either take it or are completely left behind in the damage-dealing department. The softness argument is in this context a smokescreen, yet another desperate attempt to not have to accept there are any faults in 5E
There are lots of faults in 5e, obviously.
that are severe enough WotC should be given a clear message they need to fix it.
Maybe not so much. 5e isn't sold as a neatly-balanced game to just play as-is, it's a starting point to create the campaign (and, in essence, system) you want. The message is already there in that feats are opt-in optional. Like MCing.
The notion that damage output is unimportant to martial characters is preposterous
Depending on how you use 'martial,' I guess. There are 5 arguably-martial sub-classes that are mainly about DPR, the two fighter sub-classes, in particular. Obviously, DPR is important to them, it's their best contribution to the party's success, and the only one likely to stand out at all consistently.