They can already "unpair" the phone by removing it from iCloud, as you say. The phone could simply check whether the serial number on a part is already associa

They can already "unpair" the phone by removing it from iCloud, as you say. The phone could simply check whether the serial number on a part is already associated with an iCloud registered device and if not, allow it to pair. Fairly obviously, they don't do this because it would reduce the demand for their parts, which is a multibillion dollar business.