> the only possible way to compete with that UX is to sync the private key across the user’s devices

This is my issue with passkeys. Either we lessen security to improve UX (syncing across devices implies extracting private keys from secure enclaves, at which point it’s no different to password syncing), or we have a proliferation of different keys per website across devices (assuming the website supports multiple passkeys).

Perhaps this trade off is not resolvable in a way that happily satisfies both the security constraint and the UX requirement.