But if you could back up a passkey, wouldn't the key just be a password?
(I do agree with you about backups being essential, but my conclusion was "the idea is fundamentally flawed," rather than "it's one tweak away from greatness.")
No, because unlike a password you never provide the private key for a passkey to the site you’re logging into, which is how many password breaches occur.
This is the irreducible problem. It's the Emperor's New Clothes™. So either the secrets get generated and stored in tamper-protected hardware, or they are stored somewhere else that can be made portable. For the latter, then they ought to be serializable into some standard form.
(I do agree with you about backups being essential, but my conclusion was "the idea is fundamentally flawed," rather than "it's one tweak away from greatness.")