Having WebAuthn work synchronized across devices with iCloud would be technically feasible, but it would not be FIDO certifiable which requires the private keys

Having WebAuthn work synchronized across devices with iCloud would be technically feasible, but it would not be FIDO certifiable which requires the private keys to stay on device. Typically deploying WebAuthn means that you have to put more thought into your account recovery flows - you don't want to deploy strong authentication to fall back to email or KBA-based recovery, for example. Then, this recovery may be used both to recover from a lost device as well as to just add a second device.