Skaven fight so many wars they end up losing so many soldiers they are forced to join with Archaon, because they can't stand up to him. Also, it's not entirely clear how Bretonnia is doing and what the Skaven impact is there. Skaven, from what the book shows, aren't becoming part of the chaos army, as it's more an alliance out of convenience as they can't conquer the world and fight chaos at the same time. If chaos gets beaten as they are sure to do the Skaven will likely turn on them.
Skaven has wrecked alot of places but they don't seem to have the armies to hold their conquered land. I get the feeling that if the combined Elves went after them the elves could conquer large parts of the world. I think there is a difference to beating an army and conquering a land then there is to simply win a few battles, as people can just rebuild.
Edit: Also Baron, I think the person who has the most power is who ever is in control of Averland, and they could be the next Emperor, as Averland hasn't done too badly and is one of the richest provinces, with the most powerful city, Averheim, left standing in the empire.