You changed the premise. What's being made better? The change being discussed is broken phones. That's not better. Hypothetically, that could be from a change that makes something better. Is there any reason to believe it though?
Fun fact, it'd actually be illegal for Apple to disable devices if it detects a replacement part.
There are anti-trust laws against 'tying agreements', and forcing consumers to only buy components from Apple (due to tie-in) would be violating those laws.
That said, those laws don't say anything about having to interoperate with an inferior part.
So assuming Apple isn't willfully violating anti-trust laws, we can be fairly sure the change was intended to improve some aspect of the touch controller.
Note: There are exceptions if the tying serves a purpose other than maintaining a monopoly (such as the security pairing between the TouchID sensor and FaceID camera).
I’m asking hypothetically of course...