AT&T and Verizon are definately at the top of the food chain of wireless carriers. They have the most customers and they have large profit margins. They have more borrowing ability and more capital to work with to expand/renovate their networks. There are several ways that they could mismanage their networks and start bleeding customers, but it would take a lot of churn for them to lose much ground. Verizon and AT&T have premium networks and charge premium prices. This gives them a higher profit margin. Sprint and T-Mobile have less customers and charge their customers less, giving them less profit to reinvest in their networks. T-Mobile's operators Deutche Telekom has shown little desire to invest in their network. Sprint, however, wants to build a premium network (as witnessed by NV), but doesn't have the cash to do so.
Can Sprint keep up with the big boys and attract enough customers with unlimited data to become a top dog? Or are they destined to stay as a bargain brand?
This link details why Sprint and T-Mobile will "never catch AT&T or Verizon" but I don't know if I completely agree with that assertion.
http://www.vision2mobile.com/news/2012/02/sprint-t-mobile-can-t-catch-at-t-verizon.aspx