Spectrum scarcity is also a function of our wasteful utilization. What you describe (small cells) is one version of what we call spatial multiplexing. Basicall

Spectrum scarcity is also a function of our wasteful utilization. What you describe (small cells) is one version of what we call spatial multiplexing. Basically, reuse spectrum in physically separate areas. But you can also do this in a way that doesn't sacrifice the centralized, large cell architecture. Namely via beamforming. You don't have to share spectrum if you're not dumping power in all directions. Already cell towers are split into three sectors; you could continue to increase sectors, or dynamically point beams at individual users. Being suitably isolated from one another, each beam allows spectrum reuse. It's not free obviously, it requires more expensive base station antennas, but I think a direction we'll be heading in.