I think the issue there is a matter of trust in the hardware too. What if the replacement fingerprint reader has a recorder that sends that fingerprint to the

I think the issue there is a matter of trust in the hardware too. What if the replacement fingerprint reader has a recorder that sends that fingerprint to the repair shop and the user authorizes it? Now some shady backdoor people have your fingerprint and possibly your pin and any other information that passes through that sensor. It makes sense for Apple to verify the integrity of that hardware chain all the way through when they could potentially be liable for something like that. While they could take the stance that they won't be liable for any phone with a replaced sensor, at that point it's difficult to determine where in that chain the breach happened.