Totally speculation at this point in time. Sprint may raise prices or even eliminate unlimited alltogether in the future, but that future is far away from what I can tell.
Case in point, network vision will be completed in 2013-14, at that point in time Sprint may re-evaluate the need to keep unlimited or increase prices. But i don't think so. At that point in time the whole system has been upgraded, but that does not mean everyone has gone on board and joined Sprint. I see another 6-12 months after that when you see folks joining sprint in droves since the network is completed and has shown folks that it can be among the top 2 in terms of speed, reliability and service.
With more customers coming to Sprint, they will have the cash to expand further. I assume they will buy out the smaller companies like MetroPCS, US Cellular to form one bigger company to compete with the big two.
At that point, i see them possibly eliminating Unlimited or raises prices or both.
TS