Why this cynicism?
Anonymous coward has already given a scenario where even this is dangerous.
Say, some over steals the phone, replaces touch id with one which steals finger print details, and puts it back.
The user tries to use finger print sensor, it is disabled, so uses pin to enter.
Later he steals the phone again, gets the finger prints for his use later.
In my view, any hardware changes should be handled with error 53 on a secure device. If apple hasn't done it, i would be unhappy. Say, screen is replaced. How do you know whether someone hasn't put a screen which captures finger pressing to get the pin. Or any such scenarios.
The best way should be a user setting - which says, for security purposes brick phone once any hardware is replaced outside apple stores. One who isn't concerned about security can then use as you said.
Since they didn't think about this till now, the best option is to brick them now.