No. The void entities are not stronger than the Legion. They never have been, at least not on the dimension we face them on (ours). The void lords themselves are another story, obviously.
Now they don't need to keep the Legion, but it would be a sound story move. Kind of like how the Scourge still exists but is no longer trying to destroy Azeroth. If the Legion is completely defeated, all of the content relating to demons would suddenly disappear, and that would be odd for warlocks, and demon hunters for that matter. If they became a different faction with different goals, they could still be antagonistic from time to time without being the true threat.
- - - Updated - - -
It's possible, but that would be a bit of a letdown if they just suddenly turned around while Sargeras was still alive. If we kill him and they turn, it makes sense; after all, Sargeras did find them doing that kind of thing to begin with.