However, box cutters where the weapon of choice for 9/11 you could make spears on board the aircraft and manually kill everyone except the pilots with a few burley and well trained people. Which is the real problem, defensive safety measures cost a lot and switching targets is really cheap for terrorists.
Security has different dimensions. The point of TSA security is to avoid situations where a hostile party can breach the cockpit or disable the plane in the air. All of the security theatre, random searches, etc are all there to make airliners a less attractive target and try to keep terrorism scoped.
Protecting your life as a passenger is a secondary priority, which is accomplished by the random placement of air marshals. Someone planning to fashion ad-hoc spears out of plastic utensils or whatever must account for the presence of a guy with a gun as well as passengers fighting back.
So, are you saying a vigilant public and "a good guy with a gun" are the actual solutions? (And before the reader jumps to a conclusion about my relevant political positions, I'd estimate there's an 80% chance that you'd be wrong!)
Now, it's true sometimes passengers can be... "too vigilant..." There are plenty of stories of Muslims being kicked off of planes for no particular reason, but in the end, yes, I vote public education.
For the "stop a hijacker from flying into a building" scenario, the locked door is the control.
Don't forget, we already lost an aircraft because the door locks and a pilot had a death wish. So, even if you pay someone with a gun to be on board he could be a terrorist, so now you need two guys with guns...
The society freaks out N^2(3) number of deaths involved.
Oklahoma were 168 people. 9/11 were 3000. The response to 9/11 was way greater than Oklahoma*20. It was event that defined US politics for 2 decades lead to 2-4 wars (depending on definition).
And the news cycle have already forgotten San Bernadino, Charlie Hebdo and Paris is on the brink of falling out.
Even then, it was not the number of deaths so much as the loss of a monument that really stuck with people.
Security that works by simply creating a juicier target is obviously not security at all.
The real limitation seems to be a limited number of terrorists not a limited number of targets.
That there are other targets is obviously correct, but I do not feel it really addresses the point.