in the form of a gigantic airport

Turboprops can land on grass fields... I don't know where you live but there are thousands of very small airports with just one airstrip and a small building, if the passenger flow is not very big. Exactly as there are enormous but also small train stations.

The remaining infrastructure is flight control, which is a fixed cost indipendently by where you are 'cause for obvious reasons it needs to cover the entire country anyway. And indeed the advantages of a planes is that it has very little fixed cost, so it's way easier to reduce/increase or repurpose routes at need.

The second part is, at least partly, a fallacy. If countries stopped subsidizing all cars you would see less cars but also many people unable to satisfy their transportation needs. If countries stopped to subsidized food prices you would see food waste plummetting... But also people being able to afford less food.