I wouldn't be okay with that. Say you're setting up a new iPhone with a new iCloud account because you forgot the password to your old one. (Unlikely scenario amongst us nerds, but very very likely outside of our bubble.)

If you want to log into, say, Google, but the passkey flow is the only way in, then you're almost-completely SOL unless you have some way of getting the passkey out of your iCloud keychain and into the keychain of the phone you're setting up.

If you still have your old phone, you can scan the QR code and get in that way. If you don't, then you're completely SOL.