Each building in Switzerland has an underground air raid shelter. So you wouldn't be able to kill too many civilians this way. And once you've destroyed all their aircraft, their SAMs are free to shoot an any target that's flying.
The idea is that the USA (or any other power) would struggle to occupy Switzerland, not to obliterate it from the face of the Earth. No country could defend against that, not even the USA.
i am afraid, these times are over with the advance of drone technology, look at the last Karabakh conflict. it's not just the US anymore, but lots of countries can use affordable drones an operate without much human or economic loss. Whereas the side practicing mountain guerrila tactics will have to abandon all civil economic activity, and still lose lives (you cannot stay in shelters all the time)
The idea is that the USA (or any other power) would struggle to occupy Switzerland, not to obliterate it from the face of the Earth. No country could defend against that, not even the USA.