Earlier quoted context omitted.
The least used interactions are the most important? Really? I would say the opposite, the interactions that the most users use the most often are the most important.
Edge cases are not the least used interactions, and even if they are it's not what makes them edge cases. Edge cases are defined as what happens at the edge of operating parameters the classic example of a speaker that is played at the bottom or top 10% of it's output, edge cases do not have an intrinsic definition of occurrence rarity. It might not relevant to you, but if you like blowing up your house with all that…
I think I understand the point you are getting at, design for tricky situations and the normal situations will be easier, but I think you are failing to see that you cannot design for every scenario, eventually you have to weigh up the feature sets of two technologies and pick the best one for most use cases and users, which Apple obviously thinks is FaceID.
I think FaceID will allow features simply not possible with TouchID, and expand across the Apple product line, and to other manufacturers, to the point where in 5-10yrs time we just expect screens to recognise us. This is an important step towards this.