I am less interested in the threat models than the accidental denial of service models. I've avoided passkeys because of that. It seems like there are real problems if you use multiple devices not somehow symced.
I generally add keys from two very different Password Managers to every site/app I use Passkeys with. Both Password Managers also sync between multiple devices I own.
Redundancy is already an option. Choice in Password Managers to use for your Passkeys is an option.
I understand that accidental denial of service fear, but the tools exist technically to deal with that fear if you wanted to try to deal with it.