this is part of the: passkeys as the ultimate centralized online identity plan.
not even paranoia, already happened once. just like oauth in the beginning and then everyone cried about how ugly their design was with "oauth nascar" and then only accept google or apple. it already happened once. today you can only login on saas with github (Microsoft). etc.
this passkey push will boast openess, then on day one spam will take over and will serve as the excuse to limit to google or apple attestation only. as usual. and by then it will be too late because everyone was mislead by the initial promissed openess, and like you, might even have happily added it to your service, increasing the adoption momentum.
If an app or website implements device attestation through passskeys, that's device attestation in the same way your house key is device attestation: you need the device to unlock the door.
On iOS, can pseudonymous passkeys be generated and entered by a password manager app, without being stored with the iCloud identity keychain?