This post contains some gruesome imagery and thoughts that you might prefer to avoid.
After 9/11 we spent a fortune in time and money and effort on making sure nothing like that could happen again, but in a way no one ever seems to talk about, it already couldn't, and the hijackers and planners no doubt realized this. The event itself prevents something like it from happening again. And I'm surprised I've never seen this point made.
It's not that our security checkpoints were lax in letting box-cutters through. If today you could slip box-cutters through... heck, if you could slip a 9mm pistol through, you still couldn't pull off that hijacking. And I don't just mean because of the air marshals, or the security doors on the cockpits. Even without those things you couldn't pull off the same hijacking today.
Consider, awful as it is, the situation for the people on one of those planes, confronted by a handful of men with box-cutters. The algebra of it is simple. If a few of us attacked that guy with a box-cutter, odds are good none of us would die, but some of us would get some very painful injuries, maybe debilitating ones like loss of an eye. Hijackers don't kill the people on the plane, so the choice is between the very real risk of some very nasty pain and possible lasting injury, or the inconvenience of not making your connecting flight, missing that meeting, etc. That's an easy choice: go with the hijacking. Don't be a hero.
The same situation today and everyone would realize two things, either one of which alone would completely change the formula. First: the other outcome isn't missing your connecting flight: it's probably the most horrific death imaginable, one where you might hope you died in the impact rather than the burning jet fuel and crushing building parts. Admittedly if you see a hijacker you can't be certain that's the plan, but assuming your flight is bound for a big city in the United States, you've got to seriously consider it. Second: there's the patriotism angle, the idea that not only are you looking at your own death but a literally mind-numbingly awful event. If ten men on a plane are trying to do that, maybe not everyone on the plane is prepared to be the one to help stop them at risk of their own lives for the sake of patriotism, but at least a lot of them will.
That this particular attack could never happen again was proved already on 9/11 once we found out what happened to the fourth flight. And that happened due to very incomplete information available to the people on board, and with the hijackers already firmly in control of the plane when the passengers and crew stepped up. If those passengers had found out earlier, I have no doubt that the plane would have landed safely, a bunch of passengers would have been rushed to the hospital with bleeding and stitches, and a bunch of hijackers would be imprisoned or dead. (Given how feelings were running that day, probably mostly the latter.)
I'm no expert on national security and I have no idea where to weigh in on the question of how much of our preparations are only effective against past attacks that will not come again, and how much is looking forward to attacks that haven't happened yet. Are all the precautions at airports stopping something entirely different, something that still could happen even if the hijacked-planes-as-bombs plan never could? Very possibly. But the public discourse always seems to focus on the idea of the same attack, or one very like it.