No, we should not have a "proper password" in addition to a login. People use their phones without passscodes or set them to 0000 all the time. Edit: The worse problem is if biometrics fail in that scenario, you can't access your device ever again.
In the real world, effective biometrics are the most secure login tokens we have.
So what is the problem then - they will just have face/fingerprint protection (as they have now) plus useless 0000 pin. And all this optional in settings. No inconvenience at all. Other will have proper bio + password protection that can't abused in most cases.
The problem is you are locked out forever if your biometrics fail. Requiring biometrics AND passcode means both have to succeed, Apple wisely chose to let you use biometrics OR passcode, so you can still get in when biometrics don't or can't work.
I do realize that some scenarios where bio fails exist. But I don't want simultaneous security all the time, I want an option to enable it and disable it when I want. E.g. I'm traveling through customs and I enable this mode - now the phone can't be opened by e.g. restraining me (or tricking me) and using only bio, they won't know the password to match. And if they will request to unlock my phone for inspection, well ok - but they can only look into it beside me, the minute it autolocks they won't be able to open it knowing only password that I provided because my bio is inseparable from me. Some other valid scenarios can also exist.
PS: I read that some community firmware images allow this mode, the only thing that stops me from using them is lack of camera drivers for unofficial firmware.
In the real world, effective biometrics are the most secure login tokens we have.