I'm equally surprised we don't fend off these rampant goblin threats too!

More pragmatically, such a system would cost multiple millions, and would take years to actually stabilize in a manner that would recover the fictitious costs to shutting down the airports with gaps. (i.e. I'm surprised you so easily bought into the 500k figure)

All because a bunch of idiots lost track of their one balloon, once? The asymmetry is banal. There are cheaper ways that require less planning than that.

Such a system is as easy as a set of binoculars and an airsoft gun

I avoided the 500k figure and instead just mentioned airport shutdowns.

I'm surprised you saw it in my comment. It's reminiscent of an airport seeing a would be drone.