I'll try to make things as clear as I can here, because you seem to be not understanding why I'm debating your points.
1) You claim, that Shove Prone would make GWM more potent, because it's primary way to get an advantage. That's not true.
Taking Shove Prone action instead of attack is usually a terrible idea. Even if it succeeded in making enemy prone 100% of the time, you would need to be able to make 2 (50% hit chance or less), 3 (65% or less) attacks to make it worthwhile. Level 5 and Action Surge might get you there, but the problem with a usually sizeable chance of enemy succeeding on a save remains. The sacrifice you'd have to make to get your DC high enough to make it reliable (expertise if they implement it) makes it not worth it.
As for other characters that don't get Extra Attack sacrificing their main action to make one enemy prone is just bad.
2) Saying that GWM is a valuable feat if used well, is misleading.
Even if you don't ever get an advantage on your attack, you are not making a 2-handed sword wielding character significantly worse by picking it over ASI. It's mainly because GWM doesn't even need a power attack part to be valuable. If your fighter gets 1 bonus action attack from downing the enemy per 8 rounds, it's basically on par with raising your strength score.
The reality is that if you don't know what should you pick for your 2-handed character - ASI or GWM, GWM is probably a better pick. The moments when ASI is clearly better are the ones that are circumstantial.
