"Post-9/11" began minutes after the first planes found their targets. Flight 93—the one that crashed in Pennsylvania—never made it because the passengers revolted after hearing about the other planes.
It only took a few minutes for the calculus to change. Knowing what was up, those passengers flipped from wait-and-see mode to fuck-you mode. This is pretty good evidence that you're right: the biggest increase in security was and still is that passengers will not be meek anymore.