Now how do you get the signal from those thousands of towers back somewhere to give them network access? The most common way is via copper or fiber cabling. Push enough towers out deep into every neighborhood to have minimal contention, good enough signal strength, and 99.9% coverage over the area (including all those old houses with nice thick walls that KILL signal) and you've probably spent as much or more than hiring a trenching/construction/OSP crew for a few months. And you still probably need 10-20% of those copper/fiber connections back to your headend to get to the network. And you have to provide power at several hundred locations instead of a small handful.
So why don't you just backhaul everything using multi-hop wireless? With a proper design, you're going to have one radio for subscriber use, and one for backhaul. So that's 2X the amount of equipment on every pole/tower. Then if you're over a large enough area that you can't do all the remote towers back to the main one, you're taking up bandwidth from every downstream tower coming back in addition to the upstream towers.
The amount of backhaul bandwidth you're going to need for T4->HE is the sum of all the bandwidth needed for T1, T2, T3, AND T4.
The idea has been thought of many times. Economics are the biggest reason it fails, not the technology. To get sufficient density, it costs a lot more than just running wires.