Be specific. What is something the game design clearly intends for fighters especially to do?
I can't remember what the specific term for it is, but there's a concept in game design where you put the pieces in front of the player and so long as they have a cursory understanding of the mechanics, the synergies and intended gameplay are obvious. You are playing a fighter, your class abilities revolve around attacks, -> the obvious intent is that you're supposed to be attacking.
When you roll stats, you're specialising in a certain subset of weapons. Melee for STR, Ranged/Finesse for DEX. The game explicitly tells you to make eone of the two your highest stat.
At lv1, you clhave to choose a Fighting Style. They're all presented equally as equivalent choices, and all bar 1 in thr phb enhance fighting with a particular subset of weapons, but you can only take one*. You're encouraged to specialise.
At lv4 you can choose a feat, and you read through and see that for most combinations of weapons there's an equivalent feat. They take what you're supposed to be doing (weilding weapons) and make you better at one of them. Theyre all presented as equal, and most of these are mutually exclusive. They also match well to your fighting style: and you think, if i take both ill be even better! If I team up Archery style with PAM, then I might be diverse...but at every stage you'll have a feature not being used. You're encouraged to specialise.
I say fighters especially because the increased # of feats guides you towards feats: it's supposed to compensate for low number of class abilities by letting you pick up weapon abilities.
If we then have people who have followed what the game design seems to be encouraging them to do, and then say "Wow what an idiot you fell in to the trap, shouldn't have overspecialised!"... the problem is with the game for allowing the trap, not with the player for doing what the game told them to do.
But it really sounds like your implication is broader than this - that you believe the game is designed such that a fighter that specializes in a weapon subset should be guaranteed to be better at fighting than a fighter that doesn't. That's why you think it's a trap if that doesn't occur. But the idea that's a trap is based on a presumption I don't find anywhere in the game design - not implicitly, not explicitly, not from developer communications, etc - that presumption being that fighters that take weapon specialization feats will be guaranteed to be overall better at fighting than those that don't.
Honestly yes. I don't think it needs to be said explicitly by a game designer. I think its so obvious that it's almost tautological.
It's part of the game contract with you: that if you have abilities, barring exceptional circumstances, you should be able to use them and use them effectively. Maybe not 100% of the time, but it should be the default.
Edit: to clarify, I'm not saying that PAM should unequivocally be better than any other given feat or asi; I'm saying that going from some baseline to that baseline + some other ability (feat, asi) should be an increase in power, so long as I'm doing something vaguely sensible with the build. Weapons feats are unique here because they cause some level of lock in with a different aspect of your character build.
I think that DMs should be very careful when negating or superseding a feature: if a player has effectively 'spent' part of their character on that feature, it should be contributing something to the game. If it somehow becomes optimal to do something other than that feature, its a bad frustrating feeling, especially if you cant spec out of that other thing. It can be something you use sparingly - eg flying enemies every now and then is interesting - but if you roll a STR fighter and encounter mostly flying enemies, that's a conscious choice by the dm to negate your features.
For things like GWM/PAM, they're not traps as such. They're so overturned and clearly superior to most other options that you probably SHOULD take them, but in doing so it locks you in to particular weapons and out of the "magic weapons" pillar of the game. Not the biggest deal, but again, frustrating to potentially be cut out part of that aspect of the game unless you want to sacrifice some of your power budget.
The particular example that sparked this tangent was a player who was obviously outwardly frustrated that their character was more effective using a magical mace than the polearm they had specialised in. If that was the case, then they have abilities that aren't doing anything, and fighter is short enough on abilities as is that losing any, as stated, frustrating for the player.
I'd say it goes so far as to be a failure of DMing if one player is obviously unhappy and the DM holds they keys to resolve the situation but refuses to.