I unlock my phone and then not look at it when making phone payments (Apple Pay will definitely take a step back with this). I also switch between maps and driving apps in the car while keeping my eyes on the road. I often peek at something on my screen well outside of the FOV of any front camera. My use of my unlocked phone is not at all limited to things I need to take my face's positioning in account for.
I don't particularly appreciate being accused of a failure of imagination and then having to respond to an arbitrary limited assumption of my smartphone use. It's not my imagination that's the problem here.
But the point is moot. My entire issue with it that I don't want to wait to initiate unlocking once it's in front of my face. Even if FaceID is instant, and I would accept that all my use is in the FOV of the front camera, Apple still requires me to slide to unlock the device.