1) is completely false. If you have a entirely full airport at peak like LAX, JFK or LHR, how do you add more flights? The marginal cost of more flights is another runway or terminal building, which is (tens) of billions of dollars.

The same problem exists within electricial grids. Providing 'peaker' plants which are only used 5% of the time are incredible expensive. cf how expensive electricity got in california when Enron shut these plans down intentionally in summer.