I think this decentralization and federation is what web3 is all about, without all the corporations calling everything to do with monkey pixel art that costs a million dollars "web3"
Proof of Stake will keep actual resource costs maintainable, and Blockchain does do some cool stuff for decentralization, and therefore federation. Right now Lemmy instances are tied to DNS. Hopefully in the future they will be tied to cryptographic assets. Distributed Autonomous Organizations are a great example of the potential.
Blockchain is basically a write only database. It's great for stuff like financial transactions, but it's not really necessary for something like Lemmy.
Blockchain/proof of work is indeed not necessary, but identities should not be bound to instances. That's the thing #nostr gets right and the #fediverse doesn't.