That still requires recreating every passkey, even if both can exist at once. If I want 50 sites to use a passkey on macOS and Linux, I need to go through the setup process 100 times, and all 50 sites will likely handle it slightly differently.

It also requires foresight if switching. I ran into this with 2FA as well. When I got a new phone, after setup it offered to wipe the old one, I declined. When I went to open my 2FA app for work on the new phone, it needed to be setup. I needed the 2FA on my old phone to login to the portal and setup the new phone. Had I wiped my old phone when prompted, I would have been locked out and on the phone with the help desk. This same scenario can happen with Passkeys.