I agree that all decisions made by a corporation are driven to make more money, whether directly or indirectly. Verizon got rid of unlimited data to preserve the integrity of their network but also to increase revenue, since more and more people are using more data and less minutes and texting.
Anyone that pays full buys a phone at full retail to keep their unlimited data is saving Verizon money on the phone subsidy and increasing the profit they make on the contract. Those that buy used phones through Craigslist or Ebay to keep unlimited will, in theory, save Verizon money by reducing technical support and warranty replacement costs.
Sprint, on the other hand, is keeping unlimited data with the hope that it will increase their customer base and, in turn, make more money for the company.
I am definitely not an expert on cellular evolution or history, but I would have to assume that in the beginning, unlimited data plans were a good deal for Verizon since smartphones were severely limited in what they could do and Verizon was able to charge everyone the same fee for usage that would seem negligible by today's standards. I still know some Blackberry users that only check email and browse the web and use less than 20 megabytes per month.
I have an unlimited plan and there are times, when traveling with limited access to wifi that I use it as such. I plan to keep my home internet as well, especially when I've successfully played cellular data technology against them, as a legitimate competitor to get them to lower the cost, so most of the time my cellular data usage is 0.05-2 GB per month, but I truly think data people's data usage will go up as cellular phones continue to advance, so I'll do what I have to to keep unlimited data.