They've solved that issue by deploying more towers in high density areas and carriers have swapped allocations in areas to get more continuous spectrum. Upping the frequency just gives you wider bands.

I think a better argument could be made about spending R&D on making 4G/LTE require a smaller band. I think LTE (assuming 850MHz) can be 1.4, 3, 5, 10MHz per band- which is non-optional. If you could cut the required Tx/Rx band width in half; I think that would be better use of effort.