> Assuming Lemmy will be "the winner"
there's also kbin, and you can post/read from either with any Mastodon/Pleroma/Misskey account just with a worse UI. i'd be surprised at this point if the winner isn't just Activity Pub with some thin layer on top.
> different instances already stop federating each other, thus splitting the community early.
instances usually provide at least some reason when they defederate with each other. maybe users will learn to be cognizant of who they're associating with when they pick an instance. i've seen that with tech-literate friends joining Mastodon who pick a 4chan-like unfiltered/unhinged instance as their home without understanding that such an instance causes outsized complaints compared to the "average" instance and will have less access to outside communities as a result.
i'm sure there's an opportunity for better admin/mod tooling -- hopefully some of the defeds that have happened in Lemmy recently will reverse once things settle down -- but i think even with that it has to be a new way of thinking both for users and operators about what it means to be a member of an instance. it seems that when you hand someone a magic knob they can turn between "less toxicity/abuse" and "broader reach", most will turn it further toward that "less toxicity" edge than Twitter or Reddit do themselves -- even if the tooling for it's so crude. i get why that worries people, but honestly, the benefits of that in other federated systems i use so far have been incredibly worth it (and the cost of migrating to a different instance overblown).