> Why would you choose that over a synchronizing passkey manager?

No second factor, compromised passkey manager leads to compromise of all accounts. This is a huge problem.

> A third party OAuth provider puts you at the mercy of the service provider, the other can work fully on your client side even if the app provider were to disappear tomorrow.

Yes that’s what I was saying, it needs some thought and careful work. It would need to be decentralized, and I’m not sure that current standards are up to the task.