As a user I still don't understand this. What happens if there's a house fire or something and all my devices where I'm logged in with Google break? How do I log into my account again?
Passkeys are instead of the password. You can still login using your password. This way, you don't have to keep entering your password if you have access to a device with a passkey and can access that device.
That other factor might still be available for account recoveries (together with a password or recovery email etc.), but if either are not regularly exercised, users might forget them or lose access to them and not notice until they also lose access to their passkey(s).
That said, Google's and Apple's passkey solutions themselves are cloud-synced (with no way to opt out), so as long as users of either can still access their Google or Apple account, they would not be totally locked out.