It really won’t. Mastodon has whole block lists and has done for years and guess what - user retention is no problem at all.
From what you’ve commented here, you’re annoyed that you keep picking instances to join that allow shitty things to happen and then don’t. The issue there is not defederation, it’s that:
Moderation is lacking
Your insistence on wanting to join an instance that’s a free-for-all
In the real world, instance Admin’s are legally responsible for what’s on their servers. If you want to defed from absolutely no one then you need to rent a VPS and spin up a Lemmy instance of your own. That way, the legal risk is all your own.