The pre-2001 screening was perfectly fine, and would still work today (maybe excepting the improvements to detecting plastic explosives, although I imagine that screening would have picked up the detonators just fine). The other major improvement to security was locking, reinforced cockpit doors, which should have been the standard on all commercial aircraft from the start.

What went wrong during 9/11 was that passengers assumed that a hijacking was a financially-motivated detour, rather than a political/religious attack on our ways of life (and prior to that day, there was no reason to assume otherwise). As such, there was very little resistance to the hijackers as people didn't want to get cut by a relatively small knife. Had they had even the faintest idea that failure to resist could have resulted in what happened, you can say with 100% confidence that things would have gone down differently.

Basically, that trick was good once and once only, regardless of the change in security policy that it brought about. And let's face it - only an idiot would try that again; there are targets with far higher value (and probably much more exploitable security flaws) if someone wanted to send a message again.

> only an idiot would try that again; there are targets with far higher value (and probably much more exploitable security flaws) if someone wanted to send a message again.

Specifically, the large gatherings of tightly-clustered people waiting at a security checkpoint are much more vulnerable than any plane full of passengers.

The number of times I've wanted to find a non-harmful way to prove that point (smoke grenades come to mind)... but the risk to me is not worth it, and all it would do is introduce some sort of pre-security screening that completely fails to solve the problem.