Years of poor security have taught us that we need our authentication to be easily changeable in order to be secure. It's not true. Passwords need to be changed because they can be guessed. They can be leaked. Any person sitting down at any keyboard could type any random string of characters and, given enough time, figure out someone's password. It doesn't work the same for fingerprints. There is no number of times I can press my finger on your scanner and trick your scanner into thinking I am you. Your fingerprint only needs to be changed if someone steals your finger and keeps it in a state where modern fingerprint scanners will still recognize it. That is exceedingly difficult to do.
We need to get it out of our mind that "we change our passwords regularly, we should change our fingerprints too". Bad security advice led to routine password expiration, and that bad security advice lives on. It's still bad.
>it's clear that fingerprints are closer to username than password
That is not clear in any way, either in theory or in practice. The entire argument works on "fingerprints are publicly visible and cannot be changed" which would suck for a password, but fingerprints are not a password. That's why there's an entirely different name for it. Yes, I can see your fingerprint. But TouchID isn't going to be fooled by a piece of scotch tape lifted from your desk, so it doesn't matter.
Fingerprints are neither a username nor a password. They are a uniquely identifying attribute. Usernames and passwords are not. There is no comparison between the two authentication systems.