There's also the specific case of synced passkeys, which aren't exposed to CTAP management APIs for external parties, only to the OS/platform itself. You seem tied to a narrative where a user can install a native app that gets permission to call core OS/platform APIs that let the app get all the public keys of passkeys on the device, but no such permissions/APIs exist for apps, and providing them would be in explicit violation of the fundamental security model. In reality, only the platform/OS and highly trusted actors/components that are already within the existing trust model have such access for internal purposes, and if that's not a safe assumption, it would have broader implications beyond this concern.