Good.
While serialized parts allows a record of the repairs and replacement parts on a device, which is especially nice when buying used and refurbished, the hijacking of functionality is the real crime. Screen replacements, for example, on the iPhone 14 series, even when using genuine Apple parts and without "re-pairing" by a certified technician or through SSR, results in the loss of True Tone, auto-brightness, and other critical functionality.
Here's an (incomplete) list of some of the functionality lost when doing a replacement of said part, even when parts are genuine, off of the top of my head:
1. Battery -> loss of battery health metrics
2. Display -> loss of auto-brightness, True Tone, and Face ID
3. Camera -> loss of portrait mode, buggy Camera app functionality, and loss of optical zoom
There is no technical reason why this should be the case - based on teardowns and hardware analysis even -
other than to make non-genuine parts so crippled to discourage self repair, third party repair or to encourage a device upgrade.
EDIT: Some posts below mention the "theft deterrence" argument, where serialized parts may discourage iPhone thefts by making their parts less valuable by inhibiting functionality. There is no evidence to suggest that this is an effective theft "deterrent". In fact, iPhone thefts are up and parts are still being sent to China in record numbers, even with these "deterrents".