Eh, while one would assume that there are many defense systems which nobody knows about, in the event of an actual attack lots of things would be going into the air in a very short timeframe. Best to do everything to reduce complexity of the situation, you wouldn’t want to see a civilian aircraft get misidentified or an in air collision or some other mistake.

Even if anti missile defense isn’t the reason I still see good reason. You should gain as much control as you can before the event. Putting people on the ground helps put us in control. We may have to send up fighter planes and whatever else and we don’t need to worry about maneuvering around passenger planes.

I view it like a flat tire. If you have a slow leak, you’re mostly in control and can get it refilled and monitor it. If you know the tire is losing air at medium speed, you can navigate to a safer space and potentially to a place to fix the issue. The worst case is a blowout while driving 70+ MPH on a highway with no shoulder while you’re only halfway paying attention to the road. All situations are about how your control is limited and maximizing your actions to get a safe & disruption free remedy. If you’re driving high speed and your low pressure light comes on, you’d be wise to at least pull over and inspect the issue. Not a perfect analogy but you get the gist.

Or that an airplane failing to squawk is shot down because everyone thought the country is under attack, and the airplane had a malfunctioning radio. Similar to the KAL007 situation, a civilian plane shot down by Soviet air defense during a tense moment (also involving a missile test).

Assume Los Angeles International Airport was suddenly unusable due to an attack. How many airplanes in flight would suddenly need to find a replacement place to land with limited fuel available?