But you are still supposed to have a ROM file for a game in order to play the corresponding region in PokeMMO. So, in theory, everyone playing PokeMMO already bought all the games. And this would be true for B2W2 also.
I don't think we should have B2W2 because we would not be getting any new pokemon. All non-legendaries are already obtainable, so there's no point to adding more regions unless it's gen 6 or something.
Also I'm not sure if it makes sense from an in-game perspective in pokemmo because now you have 2 Unova regions.