There is a limit to how much people can get into Cyrodill. You aren't going to see millions of people in one area. And just because everyone you know is going to Ebonheart doesn't mean everyone in the entire world is joining the Ebonheart (I know I'm not... too much Nords).
There's going to to be many different "campaigns" that cap off at a certain number of players from each alliance. This balances Cyrodiil; At least, faction wise.