That's assuming they stop collecting on that loan once it's paid off. When I worked at TMobile we'd have accounts with phones that were eligible to be unlocked, and which were eligible to be moved to a cheaper plan, and the policy was just to leave them as-is unless they said something.
They keep you on a plan but they don't keep charging the payment for the device. In my case I'm on a cheaper plan than current offerings and have three lines, all with paid off devices.
That's only true if you're on a plan that supports it (i.e. the newer ones). It's been a while since I worked there, but I bet they still have legions of old folks on grandfathered plans that are not sensitive to whether the device is paid off or not.
