When he was ranting about crypto, I had assumed that Bluesky had done another silly Coin or something. Nope, this is just apparently a dislike of public keys and signing messages, which seems like a great idea for a federated protocol.
I'm not sure why they're even using this because the docs state that the current DID stuff is all placeholders until they can find something better.
DID is there to stay, it's that the DID spec has a concept called "verification methods" https://www.w3.org/TR/did-core/#verification-methods. They are providing a verification method called "DID Placeholder (did:plc)", because none of the existing methods suited their goals. The idea is that if and when a better verification method appears, they can move to that, but that doesn't mean abandoning DIDs altogether.