While I agree that disabling the fingerprint scanner-related features would be less heavy-handed than disabling the whole phone, the fact that this ONLY happens with this very specific change to a very specific part of the security features of the phone makes it clear this has nothing to do with Apple trying to hurt the third-party repair businesses. They could easily do this when any number of other components are changed, but they don't.
So yes, their response to this tampering is extreme, but so is Apple's stance on the privacy of their users' information. They went to great lengths to develop a security system that can't be compromised, and this is part of that.
Maybe we should be pointing out that Apple is the ONLY major technology company to be taking a hard stance against the government when it comes to installing "back doors" in encryption algorithms? I mean, come on. Knocking them for the way they deal with privacy and security is crazy. I know they're a fun company to hate, and there's plenty to dislike, but on this issue—the privacy of their users—they stand heads and shoulders above everyone else.