Indeed, there's a viability threshold for a community, and it's probably on the order of 100 active users. Having them spread out isn't doing any of them any favours.
But that points to the need for and importance of discovery tools. Community tags, better search, better federation tools, better back-linking and cross-posting tools, user-defined lists, etc. The Misskey/Calckey "Antenna" saved-search feature would actually be very powerful in the threadiverse, particularly if coupled with community and post tags, and would really improve the visibility of new or undersized communities to those who are looking for them.
But forced amalgamation across independent and independently operated websites definitely isn't one of them.