I also struggle to see the issue here. People will subscribe to the various communities across instances, and they’ll quit the ones they don’t like, thereby making the best ones rise to the top, just like it works across subreddits of the same topic.
I guess the concern is discoverability? On mobile web, Beehaw’s homepage show “Local” (not sure if just Beehaw or all instance). It’s true that it’d be good if the default was “All”, so discoverability isn’t fragmented.