Server merges are great for the short term. In the long run you have the same problem need to be merged again. What a couple of us decided to do to boost Indigo population was camp out newbie areas and offer plvling even if they did not want to join guild. We did around 30 last weekend. Most of them have still been logging in. Our solution for now is to keep trying to get new players to stay. PVP servers have a bad rap for scaring off new players so helping them start out is a great way to counter this. I also have been warning them to avoid Varnas bridge unless they want to PK.
I don't see how server merges are only good for the short term (maybe true for PvP servers because new players still won't stay for long) but for PvE servers, once the population is high, end-game instances will began to be run and more guilds will be created. The population will then stabilize and maybe occasionally there will be someone who quits and someone who joins the game.