the skills learned to obtain a PPL(A), or an IR, or a helicopter rating, etc., are not really necessary to take control of a 767 and fly it into an office tower
Very true - 9/11 was more about a failing of imagination i.e. no one who was in a position to do anything about it had imagined it possible that a group of people had both the will and the ability to do such a thing.
The excuse trotted out now for the many draconian restrictions in freedom since 9/11 is that
anything is justifiable if it can prevent a terrorist event.