Even if you're using OEM parts, if you separate the sensor from the secure data storage component, that breaks the security chain and is effectively "tampering." Once you break that connection, you've breached the security encalve of the TouchID device, and all bets are off. Allowing unknown parties to disconnect and re-connect sensors opens up the possibility of someone being able to capture or interfere with the data between those parts. (Put another way, if keeping your job as an air-traffic controller means you have to pass a drug test, you nor the FAA are going to shrug it off if someone unauthorized breaks the seal on your urine sample to swap out the lid for another lid. Even if that unauthorized person was just innocently putting on a newer OEM lid, there's no way to be sure he didn't sprinkle in a few grains of oxy-something, swap out your THC-saturated pee for a clean sample, or just sneeze into the jar. A broken chain-of-custody is a broken chain-of-custody.)
So... replacing the entire TouchID chain of components would be the only way for a third party to swap out these parts for new ones without breaking the security chain internal to the TouchID 'enclave.' But then, even if the new parts were all OEM, that just creates the scenario I noted previously, enabling me to steal your phone, swap out your TouchID components for mine, and use my fingerprint to gain access to the contents of your phone.
So to prevent that scenario, the TouchID sensor and data storage component enclave have to be securely chained to the phone's other components. This means that maintaining the integrity of the fingerprint security system requires maintaining a secure and uninterrupted data chain-of-custody from the surface of the sensor all the way to the controller board of the phone. If someone not authorized by Apple breaks that chain anywhere, they invalidate the security of the device. That invalidation doesn't require proof of actual nefarious action on the part of the unauthorized repair shop. It only requires that a policy of allowing data chain-of-custody to be broken opens up the (even remote) possibility that someone somewhere could exploit that vulnerability for nefarious purposes.
Any given individual user might not care about that, but Apple has to.
Bravo sir , well put.