Sprint literally bet on the wrong technology every single generation. They also had trouble getting many popular phones because they used such oddball network technology.
You just explained why prices went up - not because Sprint exited, but because the market bifurcated into prepaid and postpaid. You can get unlimited everything on T-Mobile's network from Mint, for about $30 per month. You just have to pay up front, and you have to bring your own phone. The "majors" have raised prices on 1- and 2- line plans because they are positioning themselves as a premium product especially for families and groups with lots of lines. No point in competing to a race to the bottom vs Mint, Cricket, Metro, etc.