the thing with megamax is that they give you few options, but mfa gives them to everyoneThe reason why they call it "Megas for All" is because Mega Evolutions get granted to all different kinds of Pokemon, not so they can be used all at once in a single battle.
There's not really a reason to change this mechanic, and I think we should leave it where it is. Think of it as Megamax, where the concept is Gigantamax, but only one Pokemon is still limited to using it in battle.