Are there truly that many folks that use Verizon roaming on Sprint?! In my 13 year run with Sprint I've semi successfully made two calls on Verizon roaming and one was in a basement and that call dropped, and the other was in a super in a super small town upstate NY and that call kept failing. I just don't see the need for it anymore, if your constantly in a fringe area that ends up roaming on V, you should probably just switch to Verizon or whoever covers those places.
A lot of rural carriers are part of the CCA which is basically a lobbying group that advocates for the interests of smaller carriers. Part of being in the CCA means you get favorable roaming agreements. Currently, the combined coverage of all CCA members more or less matches Verizon's coverage. So while having Verizon may have made it easy to use them as a one stop shop for all of the U.S., it isn't like they are left with no options.
I did see that as well, but TMO is still at least 400 thousad square miles behind Verizons LTE coverage.. and some of there band 71 is still not cleared.. I don't see that happening anytime soon.. the only one that can maybe match that date is att.. if att even wants to do that