Fix the feat. Don't try to fix the people.
This should be written in stone tablets or stickied somewhere.
Problematic rules cause problems. When those rules upset the balance cart, that's a problem.
It's striking comparing Basic D&D with what happens when you add feats. Suddenly, if you don't pick from a very short list of 2-3 feats, early on, you are WAY behind the power curve of those that do. And many people aren't interested in damage feats, they like stuff like Mounted Combatant or other situational ones.
But if you compare Barbarian A with GWM to Barbarian B without it, A will be dealing close to double damage. That is by definition a "must have" feat. Meaning it's unbalanced. Dual Wielder is the opposite of a "must have", it's a "must avoid". It's a trap. Trap choices are bad for the game, especially when it's for a popular character archetype.
A GWM + PM barbarian is going to be doing 2x - 3x the damage of a dual wielder. And once your attack bonus gets up there, the fighter will be getting near free use of that +10 damage on 4-5 attacks per round. Kind of hard for other classes to compete there.
Rogues don't need to be strikers, but they need to stay in the ballpark of relevance. Without access to GWM because it's for heavy weapons only, they aren't.
I just don't see how any fighter or paladin or barbarian or even ranger, who is strength based, isn't going to end up using a polearm by level 8 or 12 at the very latest. Once they've maxed out their str, and taken GWM, they will take PM, for sure. It's the only thing to take. Really they should take PM at level 1 or 4, when GWM's -5 to hit penalty is harsh. After level 5 or so, it's time to start power attacking.
The basic combat system of the game should be balanced with or without feats used in the game. As soon as you add feats, some fighting styles become worthless. TWF in particular gets weaker and weaker as you gain more attacks, since it only benefits your bonus attack. But the polearm guy is laughing all the way to the bank, because he's got it for free. Meaning he can take the +AC style (power choice), or GWF. Making him strictly superior to the DW character. In Basic D&D this is not the case. Polearms are only situationally useful there.
The polearm master feat would be still worth having even if it only added threatening reach and proficiency. In a game with GWM or other +10 damage feats, you either jump on board or get left behind. Far, far behind.