You are correct that a Passkey ecosystem has an inherent risk of being locked out of cloud storage / sync, and that a third party escrow system is a mitigation against that. But it's not sufficient. You'd end up with keys that could, at best, only be imported into authenticators of the same ecosystem you were denied access from which, as Sync Fabrics are not interoperable. This is presumably not the outcome you're looking for.

I believe some sort of mechanism to assert credential strength at presentation time rather than generation time, and/or some sort of mechanism for TPMs/Secure Elements/Secure Enclaves to establish trust and import trusted credentials from a different authenticator vendor would be needed. This would allow vendors that don't control the hardware (i.e. are not Apple/Google/Microsoft) to build something like a "1Passkey" without having to implement their authenticators in software (i.e. a Virtual Authenticator), and you could keep your wrapped passkey store in escrow with any third party of your choosing.

You accept Google and Apple might deny you access but then you just blithely assume the US Federal Government (for example) would never do so, which shouldn't pass the laugh test.

I can imagine it being valuable if my government wants to help get me back in to, say, my bank accounts if I somehow lost all my credentials (e.g. my home burned down suddenly but I somehow escaped with nothing). But I don't feel like my GitHub, Gmail, Patreon, etc. make sense in this context. If my friends can lose a phone every year or two and make a new god-damn account, I think "My home burned down and I have nothing" is a good enough reason.

Gitlab's attitude of (for unpaid accounts): Too bad, just make another one - seems appropriate for almost everything. If tialaramex never wrote another HN comment, and instead tlrmx or tialaramex2 or whatever began posting, who would even care ?

I wouldn't want escrow of private keys, however. I'd rather the USPS just act as a certification authority that provides strong guarantees of identity verification.