Because of this discussion (and because I hadn't looked at passkeys for quite a while), I did some testing...

And passkeys integrate seamlessly into my keepassXC to get backed up with all my regular passwords. So software- instead of device-based passkeys that can be properly transfered, synced or backed up work...

The problem is that tech-minded people that use password managers and think about security and backups are not the primary target group that passkeys should protect from every-day phishing attempts in the first place. And when it's supposed to protect the common user effectively and conveniently/frictionless but then instantly destroys the effect by advising to set up another login method it makes no sense.