But unlocking the bootloader, even only once official major updates has stopped, is a risky business move due to activation lock and Find My iPhone.
If they did this, there's a good chance phone theft creeps back to high levels, since thieves could just
1. wait until Apple releases new firmware that unlocks the bootloader
2. use that new firmwaare to wipe the memory that handles activation lock
3. re-install a fresh iOS
4. resell the phone
iPhone 8's still go for over $100 on eBay, so it's not unprofitable to 'inventory' stolen phones by throwing them in a desk drawer to sell within a few years.