What are you talking about?
What fragmentation? I have had zero problems like that. Yeah, there is content that I am not directly seeing, but that is some seriously bad-quality content and if I want to fry my brain, I can always go to the home instances of the idiots and see their crud there.
The most important reason why federation is needed is that it's the only known way to keep a forum non-commercial for good. You try adding advertisements to a Fediverse server, you get an exodus of users to other servers that show precisely the same content. If you try adding a mandatory ID check for a Fediverse server, users will migrate to servers who don't have that.
Any non-federated social media will get facebookified/twitterified once it becomes seriously popular. You simply cannot sustainably run a forum without federation.