I can assure you that stolen iPhones absolutely aren't worthless. They are sent to various places where extremely technically sophisticated groups are still able to make at least 200$, sometimes much more, from a stolen iPhone, which isn't much less than what it used to be worth. In fact, phone thieves in many places only ever made about 50-100$ from a stolen high end phone.

Phone theft is a billion dollar industry, and as Apple tries to lock down parts, thieves adapted by forming far more efficient logistics and by centralizing value extraction from stolen phones. Apple's tactics haven't so far made a big impact to a phone thief's incentives, and I don't see it happening in the near future either.

I agree with the zealots but there could be better ways - for one, make the replacement screens "unpaired" by default, maybe using a WORM to store keys, so people (or repair shops) could still repair their phones with legit replacement screens while "second hand" aka parted-out displays would still be impeded.