> passkeys can be copied out of the device for safekeeping
You can't copy them out on at least the iOS, Android, and (to my knowledge) Windows default implementations.
> lots of real-world people started using passkeys and demonstrated that you absolutely must have a way to back them up and move them around.
Millions of people use them without being able to move them around in the way you describe.
> You can't copy them out on at least the iOS, Android, and (to my knowledge) Windows default implementations.
Pardon? The official support docs disagree with you [0][1][2]. They absolutely leave the device.
Other passkey managers let them leave the device in a way that you control, but even the default ones copy them off the system they were created on.
[0] <https://support.google.com/accounts/answer/6197437?hl=en&co=...>
[1] <https://support.apple.com/guide/iphone/passwords-devices-iph...>
[2] Examine the "Can I use passkeys across multiple devices?" Q and its A here: <https://support.microsoft.com/en-us/windows/passkeys-frequen...>