Given that you don't strictly need to have one passkey per site, is this desire to move passkeys around a holdover from wanting to "export" your passwords? Because if you can export them, an exploit can too. I find passkeys rather more interesting when they cannot be exported from a HSM / key enclave / yubikey, but of course I need to be able to register multiple yubikeys per site, and a few of my accounts didn't allow for this so I ended up using my yubikey for TOTP since I can have the same seed on multiple devices.