Air defense rockets came about because they were an improvement on crew controlled flak cannons. Now adays though it would be pretty trivial to create a flak autocannon that uses machine learning to absolutely mow down clouds of drones in a few moments.
Only if flak cannon is there. How far can it engage? 3km? How many you need to cover one airfield? Fuel storage? Ammo depot? How many airfield there are?
Multiply and you need a hundred billion USD of investment. And a lot of ammo which is not cheap either. Phalanx auto cannon fire rate is 3000+ USD per second.
Theres investment on either side, one to put up the defenses another to field the drones. US spending figures are misleading because they also serve as domestic job and training programs, a different story compared to a military that has to go to the global market for their kit.
