FaceID works better when your hands are dirty or unavailable (e.g. you're wearing gloves), TouchID works better when your face is covered or your look changed slightly[0].
FaceID has been a strict downgrade for me, there are very few situations where FaceID works and TouchID wouldn't have, the reverse is a daily occurrence. And things would be even worse if I couldn't work from home, as TouchID works just fine with a mask.
[0] in bed without my glasses FaceID works 0% of the time, even resetting the entire thing and re-registering my face with and without glasses, it works all of 2 days before it stops recognising me without glasses.
If a device supported and was provisioned with both, and unlocked with either, you would have double the potential for biometric attacks/vulnerabilities (remember that FaceID is an order of magnitude stronger biometric).
If you support just one with the other disabled, it is wasted hardware/space in a device with extremely limited size/cost budget. If you require both, you haven't fixed the usability issues.
Now you're taking up a fair bit of front of the phone real estate (and adding some cost) for a feature that I'm guessing most people don't care about. [ADDED: As others note there are potentially other options though that don't involve putting the home button back.]
In those dire situations I would opt for passcode unlocking with smartphone gloves. Face ID, Touch ID and passcode, only the most extreme scenarios would need more.
Also, these are not “extreme” or “dire” or even uncommon conditions - most of Northern Europe, Canada, and the north-eastern USA experience several months of such conditions every year.
Let me add to that several more anecdata: my parents could never get TouchID to work; it required them to re-register on a weekly basis. I don't know if it's because of their advanced age or physiology, but in winter, my fingers were also not reliable due to dryness.
FaceID has been quite good other than the masks issue. I just wish Apple would allow a grace period for rechecking auth.
There's no reason that a phone couldn't have touchid and faceid, other than the desire for every square mm of space to be a screen (any didn't they have touch ID on the back of the phone for a while too?)
Thats a very big "other than" virtually all users like and want to keep the full screen size. Their best next move would be to include underscreen fingerprint scanning and face id in the next iphone.
FaceID works better when your hands are dirty or unavailable (e.g. you're wearing gloves), TouchID works better when your face is covered or your look changed slightly[0].
FaceID has been a strict downgrade for me, there are very few situations where FaceID works and TouchID wouldn't have, the reverse is a daily occurrence. And things would be even worse if I couldn't work from home, as TouchID works just fine with a mask.
[0] in bed without my glasses FaceID works 0% of the time, even resetting the entire thing and re-registering my face with and without glasses, it works all of 2 days before it stops recognising me without glasses.