Even if it has to turn on the screen flash to see your face in the dark? My wife wouldn't be happy with that. And neither would I as my eyes wouldn't be prepared for extra light in many dark situations.
And if it doesn't work when dark then I'm left having to enter a passcode which I find to be an annoyance.
Now, if Apple wants to add it to supplement Touch ID I'm all for it.
They could use IR like Kinect. Check out the video below. Of course the camera on iPhone 8 would need more compact dots to see more details.