I don't know if you are saying that the smartphone market will be considered saturated once a few more people get one, or that like 90% of people will have a smartphone soon. I would not consider it saturated until all but those who will never get one, get one. Currently, most Americans have a cell phone but do not yet do not yet have a smartphone.
Have you noticed that the new, non-smartphone offerings, at least from VZW, are terrible? It is as though they are trying to force people into data plans by forcing them into smartphones.
So long as the demand for cellular data remains high and there are enough new smartphone users to satisfy the carriers, the data plan prices will remain high. I'm actually surprised VZW grandfathered-in the unlimited data plans. If they took away mine, what other options would I have?