I think that there are a few legit issues for mods who don't want to spread out, but I think that those are problems that either are going to have to be fixed at a technical level on the Threadiverse anyway or where we want to push people to spread out anyway:

• If you put work into creating a community, you don't want it to be on an instance that vanishes. Legit concern. Lemmy.world is the biggest lemmy instance right now, so "safety in numbers" -- if everyone else is there, then hopefully it is going to stay up. But (a) every other would-be mod is in the same camp too, and the only way to address that is to have people start spreading out, (b) having some mechanism for post-instance-failure community portability to another instance might be interesting, but we don't currently have it, and (c) right now I think that people look at user count and maybe community count to figure out where they should go, and so it'd be nice to have people spreading out.

• The way lemmy and kbin presently work, communities are only visible to users on other federated instances in searches that aren't specifically for community@instance if they have at least one subscriber on that other instance. However, they're visible to all local users regardless of whether there are subscribers. Setting up shop on an instance with a lot of users thus helps visibility. I think that this is legitimately a technical problem right now with both lemmy and kbin that will have to be addressed. Maybe messages don't need to go to other instances, but at least communities should -- not a lot of traffic there. Or maybe high-vote/high-traffic threads should have a chance of going to other instances. Or maybe some entirely-new mechanism to help improve discoverability of new communities should be introduced -- I don't think that either the lemmy or kbin developers are adverse to new things being implemented to improve community discoverability, but I suspect that they've had other things that they're busy with. Maybe in the meantime, someone will make an external website that tries to help users find interesting communities. This isn't fixed now, but I suspect that it's going to have to be. In the meantime, there's presently a straightforward way to mitigate this if you're a mod -- create a user account on the most-populated lemmy and kbin instances and subscribe to your community there. You can also post to newcommunities@lemmy.world, and my guess is that someone may create another community or communities for trying to promote or do reviews of or whatever existing communities. Community discoverability needs work, but everyone's in the same boat right now.