unless one just feeds you tons of ads and harvests user data. That's one reason why Gab, which is a fork of Mastodon, was defederated from most of the 'verse before Gab just went ahead and turned federation off.
You could create a Lemmy instance that made it far less user friendly to connect to other communities, and "forced" other users to join its communities because 'that's where everyone is'. That's one of the reasons why there is so much fuss over how to handle threads.net when/if they turn on federation.
I think that's against the plan with Lemmy and distributed instances, but they can improve sign up, and make it possible to migrate your user between instances, or do some unique username across all instances.
A cool feature would also be that a user could backup all their posts and votes.
I think a big help to avoid this is if any "official" apps automatically point to something like lemmyverse search or Fediverse Observer rather than Join Lemmy or any singleinstance. Mastodon.socialwas already by far the largest before the only app named "mastodon" available in the major mobile repositories was built to automatically have you create an account on mastodon.social to "Make it easier for the normies".
The fact that I dont' even know the name of any lead developers of #lemmy as opposed to /u/gargon@mastodon.social is probably a good sign too.
Simple fix, just don't join big instances, create new communities on small instances and self-host. If everybody does so, nobody has an interest into coercing users in a hermetic system, because they have far more to loose through possible defederation
We should have gotten something that's actually decentralised and P2P like Aether.
What we got was centralised servers + a glorified RSS feed that enables even more echo chambers than Reddit did... The fediverse is doomed to remain irrelevant imho