Main problem of merging would probably be item/gear imbalance. I'm only assuming, but prime should have a lot more socket sets, ruud top sets, 4th wings, full vaults of toca and so on. Some people played and still are playing on both or just have a lot of items on prime lets say, they would end up with double the items than the average medea player. This would increase the population, but what about the black market, since the most stuff goes there.
Merge should happen when both servers are at same level gear wise and when admin finds a way to fight black market. If its going to be made soon i would assume most medea players would not be happy about it, since we still have active players, competition at bosses/sieges.
For example take into account how many 4th wings and silverheart/blue eye sets there are at prime and medea, compare that and if its at least at 4 to 6 ratio, sure make it happen.
Anyway if it happens some people gonna be rich af, while the average player just gets his butt raped by 20 more players instead of the 20 current ones ๐