That's a fair point, but one wins out usually, where with the lemmy numbers they seem to remain split with the smaller communities.
I've been on lemmy for several months now and most communities are completely split and activity on any given 'news topic' (as an example) varies widely on 0-50+ comments for the same topic popping up on the feed from several identical communities from varying instances. (Which is why grouping might be an alternative solution)
Why couldn't one of the communities abandoned just be dropped from the 'group' then. Or have Moderators from several communities work together to moderate a larger one. They are still federated, just working together. If one becomes obsolete, defederate it or let it be. Why should I have to be subscribed to the same topic several times to find the discussion?