VM USA has exclusive use of the Virgin Mobile brand in the US through 2027. VM USA pays 0.25% of gross revenues annually, with a maximum of about $4.5 million in 2013 (the maximum adjusts for inflation each year based on $4 million in 2007).
If Sprint stops using the Virgin Mobile brand, Virgin will just license it to someone else in the US. A "new" Virgin Mobile USA would pop up nearly right away, which would be a disaster for Sprint. Sprint has spent jillions of dollars building the VM brand in the US over the years and it would be crazy to allow a competitor to benefit from that. If a guy says "Hey, I'm going to shoot you." you don't offer to buy him some bullets.