You know what the answer is to 9/11-style events? Locking cockpit doors. And that's what they did after 9/11. Passengers now know what's at stake too. So, while bad things can happen, another 9/11 can't happen again (and as such, the TSA does nothing to prevent it anyway).
To further this point...we can either continue to incur the cost and overhead of reacting to every terrible event that happens and changing things to prevent that particular edge case, or we can accept that bad things sometimes happen to good people and simply keep calm and carry on.
I think we're ultimately better served by the latter approach.
Also there is one actually proven method to deterring terrorist events like 9/11.
Four planes were hi-jacked. Three of those did not know that the outcome was to crash the plane into a building. But one did, and (as far as we know) the people aboard prevented that.
If the American people would simply have been informed that terrorists didn't want to steal planes, but instead fly them into target then 9/11 would have turned out very differently. And we know this because we have proof.
The solution to this threat was dispelling the notion "getting hijacked means a short vacation to Cuba" and replacing it with "getting hijacked means that I get to kick a terrorist's face in".
This solution was implemented and successfully tested on the morning of 9/11/2001.
