I fear if these federated systems do grow popular enough
If an instance did grow "too big to moderate", it would surely be defederated from, yeah? I'm struggling to think of a situation where responsible admins from well-moderated instances would willingly subject their users to spammers from an instance (no matter how big) that can't control itself.
If a Fediverse instance grew so big that it couldn't moderate itself and had a lot of spam/Nazis, presumably other instances would just defederate, yeah? Unless an instance is ad-supported, what's the incentive to grow beyond one's ability to stay under control?