or maybe at least the posts wouldn’t be replicated, but there would only be one community, and when you visit it, posts for the community are downloaded from your primary instance, as well as other instances based on some configuration that makes sense.
Just saying that 99% of users will not get this part of Lemmy. It will split all users across multiple communities where most will not even be aware of others.
I can be totally wrong, but this is my gut talking.