This level of security is perfectly achievable with an unattested passkey.

And Grandpa will have fun if he has to juggle 20 passkeys with different security requirements on his devices.

The security requirements are generally hidden from users. I have never noticed different behaviour between my passkeys.

But as I said, there are plenty of UX issues with passkeys, those UX issues however don't include getting your accounts hacked, phished, passwords lost in data breaches, etc, which I'd suggest are a lot worse.