> The lock on the phone reduces the risk of nonpayment from the borrower

This seems pretty non-obvious to me. Maybe it reduces the incidence of straight-up theft, but if someone runs into financial difficulties and can't continue paying for service, they can get more on the used market with an unlocked phone and use that money to pay off the debt/exit the contract.

Obviously that's not an outcome the carrier wants, but now we're back to locking as a mechanism to keep people stuck to the carrier.