Wasn't there a forthcoming tag-based feature to allow posts to propagate between similar communities/magazines?
That makes the most sense to me. Because then each parallel community also serves like a pseudo-snapshot of the others, allowing all others to continue without the pain of an interregnum, e.g., in the event of, say, a rogue defederation.
That wouldn't account for cultural differences between instances though. Where those are drastic, then people should get to choose which one they want to join.