It’s very difficult.
There are MANY problems. For example, all that CIWS 20mm AA that goes up in a city, comes down in a city. Doing damage and killing people.
If you put the AA in a belt around a city, it has to cover 360* around a very large area, because the drone swarms are designed to come from all directions with the same time on target, not just from the direction of launch like, say, WW2 German Buzz Bombs. So you can’t just put your AA on the “threat” side of a city.
So you would need 30 or more CIWS (at least) around every city. And a “burst” from the gatling-gun CIWS would cost far more in ammunition than the $20K drone.
They are going to need an entirely new type of AA defense, spread across thousands of square miles, and that will take time. Each defensive munition must cost less than a drone, or it’s a net win for the attacker even when a drone is sometimes shot down. So expensive MANPADs like Stingers are out.
It has to be a super cheap acoustic-homing MANPAD, perhaps, and that will take time to develop. Low-altitude acoustic MANPADs with a network of acoustic sensors. They don’t need a high ceiling or speed, these drones are flying at Cessna speeds.
Anybody who thought the Iranians were stupid, that they would try to match against our strengths, now knows better. They came up with an entirely new niche of attack drones where we don’t have a defense ready, not at all.
Maybe we need SpaceX to take on defense contracting?