Mostly this is a consequence of laziness rather than a proper attempt at serialisation. The “calibration data” for most small sensors in Apple devices is stored in a centrally in a crypto blob to provide guarantees around component combinations, with sensor serial numbers used as lookup keys. It not usual for the sensor calibration data to computed on a component-by-component basis with calibration blob being computed _before_ the machine is assembled, based on the serial numbers the machine should contain.

So adding a new sensor means new serial number, which means the data lookup now fails. Resulting in the new sensor not working at all.

The pre-computed calibration blobs are neat little manufacturing trick to provide an end-of-line QA check, proving that a specific machine only contains the specific components it’s supposed to have. But it means the setup has no proper fallback mechanism for generating new blobs outside of the manufacturing process.

I personally think it’s a travesty that Apple hasn’t properly addressed this issue and enabled proper 3rd party repairs. But I think it worth recognising that the serialisation mechanism doesn’t exist primarily to prevent repairs, it exists to provide a form of cryptographic integrity check of the manufacturing process. Preventing repairs is just a “happy accident”.