I don't live or work or even sit down in those places for very long. But they do exist. Sparsely populated, rural areas of Kansas and Nebraska will occasionally show Verizon roaming for me on SCP. In emergency situations that can be a bummer. Rare, admittedly, but not impossible considering the players involved.
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.