It's clearly a compromise, but if we were going for a perfect implementation then everyone would have a local/self hosted soloution. This just means in the case of a bad vendor/unwanted default/change to a better vendor you can migrate away from them quickly (and not need to visit them again) without having to update all your pass keys at once.

I'd expect good vendors to be popping up "You haven't updated your passkey for this site since you migrated to us, you should go to your account settings and do so" on login to a site.