Unless you want to re-log into and re-enroll every service you ever signed up for when you get a new device (which is when lots of people get rid of their old device, it’s even what device manufacturers recommend, see trade-in prompts when you buy a new phone), you do need to migrate credentials.

> The number of use cases where you need to "make a copy of " or "backup" your passkeys is zero.

That just tells me you haven’t thought about the problem at all.