The optimal thing to do in combat, aside from disrupting enemy caster concentration spells, is always to concentrate your damage on one foe until it dies. Take enemy pieces off the board to deny them their next action.
So you attack once with your d10, if not dead attack again, repeat a third time. You could choose to change up which attack you put on which enemy depending on how "peachy" you think they are getting, but unless you know the exact HP totals of your foes, it's very rare for it to matter. If you're talking about one shot kills, then the +10 damage from the GWM feat will cover that. Dual Wielder is strictly surpassed, even against lower HP enemies.
Don't forget the extra OA attack from the polearm master. That's damage the dual wielder will never get. Also the dual wielder will sometimes not even be able to reach an opponent due to limited movement rate, when someone with a polearm might. Especially when running around on the battle mat and whacking minions. Polearms kick butt in 5th ed. Way superior.
You are right that there's a rare, tiny, edge case between the time you pick up GWM and not, or when you don't use it (high AC foes), that you will default back to the lowly "dual wielder" damage. But that's a good base damage to have.
Also, don't forget, a fighter doing action surge, gets 4 timess 1d10 + 4 + 10 by level 6, plus an extra 1d4 + 4 + 10. If that kills enemy number 1, he can take a reaction to swing an extra 1d10 +4 +10 attack against his neighbour, then he can easily move around because him having reach means he doesn't need to enter melee range and thus avoid OAs. Then he uses his bonus attack to mop up. Then the next round? Enemy approaches, wham, free 1d10 attack. Then another 2 1d10 attacks, it's not hard to play a game of Go with the battlefield and use the momentum + reach to just destroy everything.
Polearm + GWM masters also get to do this kind of dance where they estimate the remaining HP of foes, attack the one they think they can kill first which is standing next to another foe, use the reaction attack against the next chump, then continue attacking that one from 10 feet away. If its turn starts and is still alive, it will probably approach you and provoke (or approach one of your allies, also possibly provoking), possibly killing it before it can attack you or them. Win.
The power and battlefield control of threatening reach and multiattacks with a super damaging attack each time, is possibly epic level in power if you're comparing it to what a 4th ed fighter could do with his at-wills. Then you can also add on maneuvers for tripping if you're a battlemaster, and double down on the beat down.
It's pretty clear that strength fighters are not underpowered in this edition. I guess they did that right. But I just think the bonus extra attack is above and beyond overkill. It's way, way too good.