UAV's aren't cheap. If you want a cheap air based solution then get an Apache Helicopter and a rocket pod filled with APKWS Hydra rockets and some Hellfire missiles just in case something actually dangerous pops up.
Do you know what's even cheaper than that? Good old fashioned bullets. Artillery and armored fighting vehicles will never die out because they don't cost ten thousands of dollars to operate per hour.
When you are holding onto ground for 24/7 you can't afford such an expense.