Static defenses are incredibly vulnerable, to drones, to sabotage, to spy networks etc. etc... a spy telling you they saw an AH64 fly overhead tells you nothing, helicopters can also resupply in improvised unpredictable locations negating pervasive satellite surveillance/other kinds of surveillance.

A static cannon cannot optimize the angle of fire to maximize safety and ensure there is no civilian collateral damage, a helicopter can take the cannon and move it three dimensionally which vastly changes the nature of the tool. **A MASSIVE safety issue with firing high explosive rounds into the sky is that they land eventually, a helicopter can position itself so the background terrain where shots that miss the drone will hit is easily visible and monitorable for making sure to not put anything at risk, and also for records keeping and munitions cleanup/safety evaluation after an incident, you have very clear footage of everything.. you can go out with a detector and walk that part of the field.. etc..

The static cannons become defense nodes that mobile turrets operate out of.. you know how this goes. It makes sense to make both use the same turret if possible.

Helicopters can be used to evacuate victims.

Helicopters are widely useful in war and peacetime. So is all the associated supply and maintenance chain.

For sensors, jamming, targetting etc... a helicopter 200 meters overhead has an immensely more far reaching picture of the area than a trailer mounted turret in most cases, the helicopters can lift more sophisticated sensors than drones and thus actively use terrain to mask and alternatively expose itself to wide areas and hide behind cover when waves of jet/rocket flying bombs enter the area (which is where the f16 comes in). The helicopter in this event can utilize traditional air to air missiles to down the high threat drone from an oblique angle from "behind cover". The helicopter while doing all of this can serve as a uplink/downlink relay to interceptor pilots.

Your helicopters are in large part for unpredictably moving your lightweight static air defenses around to again, minimize the value of being very precisely surveilled.

If the public begins to associate helicopters with air defense they help people feel a sense of safety when they fly overhead even when they are doing nothing but patrolling. The inherent visibility of course magnifies the effect.