Of course in a magical world all the different basically the same magazines on different instances would get merged seamlessly in the UI with posts all somehow connected.
In the real world I can't even conceptualize how you could handle moderation (or a random collection of posts vanishing from a thread) with instances that have different rules unless each magazine for different topics was a separate silo.
The natural outcome, without much active effort, seems likely to be that niche stuff is consolidated on a single instance that members aren't necessarily on in order to have enough participation while popular stuff truly just gets silo'd into different self-sustaining groups that talk about the same stuff but with different culture developed, different moderators, and a different instance.
Is there another way? We're assuming each one is self-sustaining, it will be good enough or you'll find one on a different instance...
I expect people will see there's existing local magazines for a bunch of things, and then to search out magazines for niche interests.