Spreading communities across as many instances as possible is no doubt a good thing, but it doesn’t really solve the forking problem, and “the community can fall back on x or y to figure out what to do” demonstrates that pretty well - if a third instance is set up to replace those two communities, then that third instance breaks down, which of the two (or, let’s be honest, more than two) different instances/communities are used as the fall back, and how is that communicated to users who likely don’t even understand federation?
For federated communities to win out over monolithic platforms, they really need to reduce the power held over communities by the instance administrators - seamless migration of communities and user profiles between instances is a major gap in Lemmy - and make it almost completely transparent to the user. The user shouldn’t really at any point see much of a difference between Lemmy and Reddit, for example.