Blockchain does not mean decentralized. There are completely centralized blockchains, and there are completely decentralized systems without blockchains (like Tor).

Centralization is as much a human problem as it is technical, that’s why everyone joins mastodon.social instead of looking for a smaller instance or spinning up their own.

As for a more decentralized Fediverse, I think https://holos.social/ is a super interesting idea.