I know I'm late to the party, but any additional usage of WiMax is going to cost Sprint money. With Sprint dumping billions into their LTE network, they can save money by enticing upgrade-ready customers to wait to upgrade to a LTE device and use their older device for WiMax until their area is upgraded to LTE instead of paying for a subsidized device that will use both technologies.
Or you can look at all the people who gave up 4G WiMax to get an iPhone. It would be similar to someone giving up 4G WiMax for the very latest Android device with the promise that they will be able to access LTE as soon as it is available. Sprint still has the WiMax E4GT for sale which is a good phone, possibly the second or third best phone in Sprint's lineup right now, but buying that will keep one from using LTE until their next upgrade unless they buy a phone off contract.