The evo 4g was just one of those phones that everyone had to have. The evo 4g was the first sprint phone I have seen so many other people with other than seeing random att/verizon iphones. When I had an apache, mogul, touch diamond, touch pro or touch pro 2, it was rare for me to see another user with one of those phones but after the evo came out, every single day I would see multiple people using it as well. So as far as my opinion as to why why things went down hill so quickly is because of the popularity of the evo. When going from my touch pro 2 to the evo my data consumption literally tripled from one month to the next. Now you can only imagine how much more those iphones will add to the current problem.
As far as subscribers though, one thing thats almost impossible to calculate is customer retention. Lets imagine for a minute that sprint never released the evo 4g, lets also imagine that sprint didn't carry the iphone. How much do you think subscriber numbers would have decreased? I can't say for sure but with out the evo and iphone to keep current customers happy, sprint would be in even more trouble than they are in now.