Once the 3G network is dead we can reuse that low-freq bands for 5G with similar range. You only use the high-freq high-bandwidth radios in populated areas.
https://youtu.be/0faCad2kKeg this is Wendover Productions explanation of cell service, there's an error regarding ultra high-freq when he means ultra low-freq, other than that it's a good explanation of how RF works.
An important bit here is that 3G is 16 combinations and 5G is 1000+, this is why it's important to deprecate old tech, we need to reuse frequencies.
You only need lots of towers if you want blazing fast speeds everywhere. Out in the country they won't be putting up towers every mile.