I would say it's not really about the immediate overpopulation at the moment anyway. The current population of Asura is very much sustainable. The issue is that if they unlock it, everyone will pile back in and the year of progress they've made vanishes overnight(in a week, month, whatever).
If they're not going to merge servers or force anyone to move, gradually closing them helps the medium servers gain population as well so there are more viable servers long term.
Most other servers are dead and very inactive compared to Asura and I dont want to permanently leave it
This part isn't really true any more. Odin and Bahamut are both up to 2/3 as much AH volume as Asura with considerably less RMT/merc activity. Granted, that doesn't help much when you can't move to them either.