The self-repair scheme is a facade, they charge just as much for you to do it yourself as they would charge to do it for you.
As far as touch/face-id, all you have to do is have the registered fingerprints tied to the sensor. If you switch sensor, then finger/face needs to be re-registered. In fact, I think it already works that way, but with the added unnecessary step of getting daddy cook’s kiss of approval in the new sensor.