The potential user base is massive once you move away from the urban centers in the developed world.

Take a look at Lagos, Nigeria. Population is ~15m for ~80k towers. Only 1.4k (1.7%) of them are 4G LTE with the remaining either 3G or CDMA.

Or the State of Sao Paulo, in Brazil (home of 22% of Brazil's population and 33% of the country's GDP). Approximately 573k towers, 76k of which are 4G

For comparison, the greater Boston area has ~107k towers and 58k are 4G.