"The question of course remains: how long can Sprint continue to offer unlimited data for smartphones?"
And it must be a big one for you Karl, as you seem to have been asking it every week for the past two years.
So here's your answer: For the foreseeable future.
Yes because their service is so robust now. Yes when all the data hogs who want their 100 GB a month mobile data bring Sprint's network down to less than dial-up speed well see how great unlimited sounds.