OK I think you mean the player base on a particular planet and not for the entire game. If not then my response is different.
I was not assuming the planets will be empty. If these guilds are placing their workshops on them t1-t2 planets, then they are not just vanity builds they plan to use them therefore the planets are not empty. New players will still start on the t1-t2 so they are always needed. Merging planets with settlements that are in specific biomes and locations possibly close to portal hubs or friends does not really seem possible to me. You cannot just scatter builds randomly across a āmergedā planet. Well I guess you can, but you are going to anger a lot of people that believe this is a persistent universe and their builds were safe.
You could say the t6 are empty should we not have them since fewer people build on them? But we need the higher tier for the resources. And there are people that like the challenges associated with the higher tier worlds.