Otherwise, if we have a lot of medium sized instances but the most popular communities are hosted on just a few huge instances, doesn’t that defeat the purpose of distributing load across many instances?
If that’s the case, how do we solve the cumbersome user experience of having to subscribe to the same community over and over again across a ton of medium instances?
This is what has already happened to some extend, but communities are free to move away to another instance. Fx !Android!android@lemmy.world moved to !Android!android@lemdro.id.
You could also say that the moderators of communities that exist on multiple instances, have a certain responsibility as well, but it is tricky. Beehaw.org has many of the same communities that the rest of the Lemmyverse has, but they also defederate with the biggest instance lemmy.world.