That's exactly what we're all asking :)
I feel like you're arguing that the tech itself doesn't require platform lock-in, and I absolutely agree with that. This is a policy issue, not a tech issue (well, attestation, but that's a different conversation). But it's policy issue that's shared by all of the major companies involved in the FIDO alliance and they're very suspicious against offering any policy solutions or requirements in the passkey specification itself.
There's no technical reason at all why Apple couldn't support syncing passkeys to a local backup that can be imported by other authenticators. From that perspective you're completely right.
They just don't.
In terms of policy, I am not sure all the players share the sentiment. Google stated they understand the importance of passkeys being cross platform. So at least, if their statement is to be trusted, they’re on the side of consumers.
I'm mostly down on the passkey ecosystem and who's in charge of it. I guess I don't do a good job of clarifying that always.
There is a theoretical alternate world where the FIDO Alliance approached this differently where I would be 100% on board with passkeys and would be encouraging everyone to switch to them. I'm just really worried about the implementations I think we're going to actually end up with.
Why do the blessed vendors get to do that but not 3rd parties? Syncing your passkeys is just copying the DB to the cloud and then down to another device. If you made a copy of your icloud db, and icloud let you import passkeys (does it?) then you can absolutely make a copy of your icloud keychain and backup/restore it as needed. I don't really understand your point here (everything else checks out though).