> But there's just no way to do that across multiple vendors.
Sure there is!
Since some superior body (FIDO) needs to declare which are the valid (or "most valid") attestation certs, not unlike the web PKI, and attestation (at higher levels) "demands" that keymat be held strictly secure by the attesting device, the CXP can be modified such that if the keymat passes between two attesting (and "arms length") authenticators, both at the same FIDO MDS level (or receiver greater), a bit is set in the exchange which tells the new/receiving authenticator to include ED in the attestation statement. That ED will indicate that the keymat passed from the original authenticator to the new authenticator, securely ie without ever being wrapped in a way that the user themselves (thus malware) had access to wrapping keys. This ED could include an original attestation as well, or not. Probably not since it wouldn't be against a nonce. Anyway, the RP can see this ED and then accept the new attestation even against a different one at registration time.
Of course this requires use of CXP and not some proprietary (frictionless) self-sync mechanism. Making that CXP "friendly" is a bunch of work, the actual hard part IMO. There's a lot more I could go on about but everyone will have lost interest by now.
Apple doesn't care about portability, the opposite in fact. Claiming that as the reason is quite a stretch. But I'm not speaking to that, just to the possibility (very much so) of such an interoperable sync between attesting but disparate authenticators.
Yes, an RP can require attestation, and yes an RP can require that attestations be FIDO MDS certified, anyway. What Apple has done is made their own job easier, out of laziness I say, because without attestation, within their own ecosystem they now don't need to create "Security Worlds" where they can attest (with same attestation cert) across synced devices. This benefits Apple, and actually Apple users, since users' keys are safe/secure if using Apple authenticators, whether attested or not, and whether RP cares or not! But the weight of Apple means that relying parties have to give up on attestation since Apple doesn't do it.
Using attestation wouldn't preclude software authenticators such as ye olde password managers, so it wouldn't force a lockout of small tech that other sibling posts are claiming are Apple's virtuous motivations. As you are correctly noting, RPs can require attestation -- well, RPs can also not require attestation if your data is not all that precious. Just as some large U2F RPs didn't require attestation. (UX, not attestation, was more the issue for U2F.) FIDO can still make a strongly worded recommendation to keep attestation optional, but eg let users know they are using an "L1" not an "L2" authenticator. Apple could have pushed to make that happen, instead of pushing non-attestation. So I still submit they took the easy/lazy way out.
> perfect replacement
hardly. at least not so far as has been realized today. (perfect is too strong a word)
Sure there is!
Since some superior body (FIDO) needs to declare which are the valid (or "most valid") attestation certs, not unlike the web PKI, and attestation (at higher levels) "demands" that keymat be held strictly secure by the attesting device, the CXP can be modified such that if the keymat passes between two attesting (and "arms length") authenticators, both at the same FIDO MDS level (or receiver greater), a bit is set in the exchange which tells the new/receiving authenticator to include ED in the attestation statement. That ED will indicate that the keymat passed from the original authenticator to the new authenticator, securely ie without ever being wrapped in a way that the user themselves (thus malware) had access to wrapping keys. This ED could include an original attestation as well, or not. Probably not since it wouldn't be against a nonce. Anyway, the RP can see this ED and then accept the new attestation even against a different one at registration time.
Of course this requires use of CXP and not some proprietary (frictionless) self-sync mechanism. Making that CXP "friendly" is a bunch of work, the actual hard part IMO. There's a lot more I could go on about but everyone will have lost interest by now.
Apple doesn't care about portability, the opposite in fact. Claiming that as the reason is quite a stretch. But I'm not speaking to that, just to the possibility (very much so) of such an interoperable sync between attesting but disparate authenticators.
Yes, an RP can require attestation, and yes an RP can require that attestations be FIDO MDS certified, anyway. What Apple has done is made their own job easier, out of laziness I say, because without attestation, within their own ecosystem they now don't need to create "Security Worlds" where they can attest (with same attestation cert) across synced devices. This benefits Apple, and actually Apple users, since users' keys are safe/secure if using Apple authenticators, whether attested or not, and whether RP cares or not! But the weight of Apple means that relying parties have to give up on attestation since Apple doesn't do it.
Using attestation wouldn't preclude software authenticators such as ye olde password managers, so it wouldn't force a lockout of small tech that other sibling posts are claiming are Apple's virtuous motivations. As you are correctly noting, RPs can require attestation -- well, RPs can also not require attestation if your data is not all that precious. Just as some large U2F RPs didn't require attestation. (UX, not attestation, was more the issue for U2F.) FIDO can still make a strongly worded recommendation to keep attestation optional, but eg let users know they are using an "L1" not an "L2" authenticator. Apple could have pushed to make that happen, instead of pushing non-attestation. So I still submit they took the easy/lazy way out.
> perfect replacement
hardly. at least not so far as has been realized today. (perfect is too strong a word)