That theory makes sense & I don't see why Sprint won't buy what it can of 600 Mhz, but the question is how much can it get and that leads to "what-ifs".
It'll add to capacity which will need to go up not down with more subscribers... If they can't get enough 600 Mhz they may not be able to move ALL data there: As people are talking less& less and using data more & more..In three more years Im sure fliphones will be dead.. right now 3 or 4 outta 10 still have them.. It might make more sense to use both bands for data.. from a quality point of view there is very little difference between 600 and 800 Mhz it'll add to capacity in areas where there isn't enough 800 Mhz ... 600 Mhz could help fill holes in major cities like NYC, Chicago, LA....where business class accounts must have the highest quality present..
While its possible - I'd think you'd be wasting bandwidth if you used 800 strictly for 1xAdvanced..we don't need all of the 800 Mhz spectrum for HD Voice because people talking less every year