"Space Data claimed it could save a new national wireless player $2bn, or construction of 8,500 new towers across the country,"
If you have the same amount of spectrum, then contention just got 8,500 times worse.
HSPA uses 5MHz per sector, or 15MHz per mast. To get near BB speed LTE needs to use 20MHz per sector, or 60MHz altogther.
Let's be generous and say the HAP has 400MHz. Covering the equivalent of 8,500 cells means 8,500 more people on average. Be generous and say the averaging is better on a big cell than loads of little ones, so call it 1000 times.
so per equivalent cell area we now have 400/1000 = 0.4MHz max. 24 times WORSE than 3G/HSPA and 96 worse than LTE.
Such a system is ONLY any use for a small number of emergency Workers, if accessing Internet as well as voice only about 1 simultaneous user per MHz of Spectrum.
In storm conditions it's useless. Storms has always been the problem for HAP. You need them at about 70,000ft even so.