One other thing is that you can bulk create your own instances, and that's a lot more effort to defederate. People could be creating those instances right now and just start using them after a year; at least they have incurred some costs during that..

I believe abuse management in openly federated systems (e.g. Lemmy, Mastodon, Matrix) is still an unsolved problem. I doubt good solutions will arrive before they become popular enough to attract commercial spammers.