I don't think there is any point in rolling out 800Mhz only though. It would be a least band 25/26.
A quick expansion of B26 800Mhz 5x5 LTE data and BC10 800Mhz 1x voice using microwave chained back-haul to locations where high bandwidth wired back-haul can be provided quickly and for less cost would be ideal in areas where there is no native LTE or partner LTE coverage. Sprint should put a site every 20 miles or however far apart they can get the 800Mhz signal to travel with no LTE drop-offs between sites. This expansion should occur along every road/highway that has 2 or more lanes in each direction where there is currently no LTE coverage for Sprint customers. This would eventually pay for itself with additional customers and elimination of roaming fees in these areas.
When the expansion covers every road/highway that has 2 lanes in each direction and more money for expansion is available, Sprint should then move on to every smaller village and town starting with the largest population sizes where they they do not provide coverage along with the roads that link these smaller villages and towns.
Any 800Mhz only site that is regularly overloaded should get additional back-haul and B25 1900Mhz 5x5 or 10x10 LTE data and BC1 1900Mhz 3G voice added to it. In any area where it is still overloaded after having both B26 and B25, should start to get filled in with B41 on the existing sites and then additional mini-sites in between. This includes any area where the population can suddenly increase such as airports, stadiums, beaches, convention centers, fair-grounds, etc.
It seems today that Sprint is focusing primarily on the B41 in-fill rather than the bigger picture. Unfortunately they are now being portrayed in advertising by their competitors as having the smallest network which is hurting them in their attempt to gain additional customers. I think there is room to have some focus on rural expansion while the majority of their work continues on urban/suburban in-fill.