They can increase the number of layers until everyone gets in, but then players will stay on the overpopulated realms. It’ll make it more difficult to turn off layering later on.
I think their current strat is to wait for the current realms to fill up and become too full (i.e. there will be really bad queues when they try to remove layering). At that point, open up a new realm. Let players redistribute. Repeat.