Monsters are there to force players to move closer.
For all I saw, they're just moving away... from the continent/game. Also, even more populated realms are facing the monsters. If the only way to avoid it is to have just two/three realms, maybe would be better to just put an announcement in the front page of BM letting everyone know it. Or, you know, smaller maps.
Even if you put such announcement, I still doubt people will just leave if you let them decide. They will decide for stay and fight till the end and then leave instead to move to another realm. After all, who will decide wich realm should be more populated? With smaller maps, the devs do the cut and you follow or you leave.