The restriction is there for a reason, it's not like they're doing it for fun. If the population on Aether became even more skewed to Gilgamesh and Balmung than it already is, I'm sure that datacenter would see even more crashes and connection problems than it already does. Expanding server capacity isn't simple and certainly not cheap.

If you really want to get into a specific server, and particularly Gilgamesh or Balmung, a paid transfer is pretty much your only option.