Yes, it replaces something you know with something you have and something you are. The hypothetical scenario you describe is a downside.
As to your other concerns. Registering a passkey from each of your devices should be a trivial exercise. If you want to play with this now, GitHub has good integration (you can keep your password, the passkeys you register are just alternative ways to access).
For your family, your concerns are not warranted because the “cloud backup” is part of the integration. With Apple, as an example, the passkeys are tied to your keychain, and changing devices/disaster recovery is relatively trivial. The passkeys are also accessible from all your Apple devices, so you don’t have to create separate ones.
As to your other concerns. Registering a passkey from each of your devices should be a trivial exercise. If you want to play with this now, GitHub has good integration (you can keep your password, the passkeys you register are just alternative ways to access).
For your family, your concerns are not warranted because the “cloud backup” is part of the integration. With Apple, as an example, the passkeys are tied to your keychain, and changing devices/disaster recovery is relatively trivial. The passkeys are also accessible from all your Apple devices, so you don’t have to create separate ones.