Forgive me ignorance, but wouldn't backends needs to store a "hash" of your fingerprint to validate? This essentially means a single giant password.
2. A fingerprint hash isn't a cryptographic hash because you need to be able to match to nearby matches. A small variation in input needs to have a small variation in the hash so a distance function can be applied.
Those are terrible properties for a password.
Would you create a rubber stamp of your passwords, slather it with oil, and then go around stamping it on everything you own and everywhere you go?
Many builders/carpenters/etc will tell you this is not true. People who work in abrasive environments sometimes without proper protection often temporarily have no fingerprints as they are "warn off".
Many injuries can effectively modify or remove the too, at least temporarily.
This makes them bad usernames as well as bad passwords.
Do they come back in the same form as they previously were?
Additionally, I was under the impression that some fingerprint readers looked at the blood vessels rather than the actual prints. Not sure how that would be affected by abrasion. Perhaps this is a misunderstanding on my end.