My concern is because regardless of what communities I can access, Mbin is so tiny compared to Lemmy that I feel a bit bad abandoning the less-used Fediverse platform. One less not-Lemmy user, even if I still prop up the same communities with my activities. Well, I'd still be a not-Lemmy user, but I guess one less Mbin user, which I feel could use all the help it can get.

I think this could and should be true. But enough people scroll All that a more community focused dynamic does get dissolved. At least so I fear.

That being said, I feel like all threadiverse platforms could go further in enabling communities to be more well defined spaces.

The private and local only communities features coming from lemmy go toward that I think. But other things like multi communities, wikis, chat, more specific reminders and perhaps visibility options for each post could help too.