If you lose your bank passkey, (e.g. if you put it in the wrong password manager and you can't figure out where it is) you can just sign in with your bank password.
In the worst case, banks actually don't make it very hard for seniors to reset your password/passkey; just show up at a branch with photo ID, your bank card, and your PIN, and a teller will help you reset your credentials. They do it all the time.
And, remember, seniors could also put a randomly generated password into the wrong password manager. In that case, they'll either have to reset their password, or they'll have figure out what they did, retrieve their password from the OS password manager, and transfer that password to their preferred password manager.
The exact same story applies to passkeys, except, because passkeys can't be copied and pasted, you'd have to figure out how to use the finicky app-to-app transfer system ("Credential Exchange Protocol"). That's probably too complicated for most seniors, so falling back to a password is almost certainly their best bet.