Perhaps I should rephrase my question a bit to: is the existence of multiple communities on a given subject a feature of Lemmy (perhaps even unique to Lemmy) we should expect and embrace, or do you think communities coalescing into few/one will occur naturally?

My take is that Reddit, Lemmy, and any system that allows non-admins to create subreddits/sublemmies/communities/whatever pretty much plays out similarly:

• Overlapping communities are a feature of lemmy, but also reddit.
• They are not unique to lemmy.
• People DO embrace overlapping communities to work out differences in moderation policies, to escape annoying culture, to achieve a smaller/cozier feel. But all this is hard work, and generally... unless there's a reason to do extra hard work to maintain a smaller duplicate community...
• Communities coalesce into few/one naturally.

I don''t feel like any of this is really different in the fediverse, the only difference is that the community name is longer tech@lemmy.ml instead of /r/tech. But tech@lemmy.ml and tech@beehaw.org isn't functionally any different than /r/tech and /r/otherTechSucksOursIsGood. The social dynamics that determine community participation play out in almost exactly the same way in both cases.

The few exceptions are with a lemmy instance that doesn't federate to any/most instances and has limited account signups. That sort of lemmy instance could create intentionally separate communities that are really tightly controlled. So you could talk about tech news exclusively with computer-science students at your university or something. But at that point it's less like lemmy the fediverse app and more like a standalone bulletin-board system like phpbb or something. For almost all lemmy instances and almost all communities on them, overlapping lemmy communities behave very similarly to overlapping subreddits.