PieFed can group communities (including Lemmy’s federated communities) by interest, it’s just not mature enough replacement but I’d wage on it being a lot more convenient for people less knowledgeable about tech to use it than Lemmy, when it gets mature enough
I never understand people comparing Mastodon to Facebook, it feels more like a proper modern evolution of what Twitter used to be than of Facebook, you can’t have communities on Mastodon which is a key thing Facebook has that Orkut also had which when it closed down there was a huge exodus to Facebook, not to say it’s like wow so important but they both also had integrated games be that Flash games or Unity Web (no longer a thing either of the two but Flash games can be played safely through emulation with Ruffle (which is open-source) and not enough games yet but there’s ongoing QoL development for web game export with Godot) again games are not that important and communities feature is a huge thing, people using fediverse thinking who wants communities will join forumverse platforms like Lemmy is not being realistic with your target audience if you want to keep talking about exodus of all kind of people to fediverse platforms, there are a lot of people who barely understand what domains or directories are
Just defederate simple as that, no need to wish others isolated just because you don’t want them, that’s the beauty of the Fediverse