What if the manufacturer didn’t do this to make everyone’s ECU better, but specifically to discourage users from buying third party parts?

I’m asking hypothetically of course...

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).