The problem on 9/11 wasn't that terrorists got box-cutters onto planes. The problem was that we were operating under a given set of assumptions and the terrorists found a way around those assumptions to accomplish their goals. Up until 9/11 everyone had a set of assumptions about hijackings: Comply with them, you'll fly someplace you don't want to go, land safely, then the hijackers will make demands which may or not be met, the hijackers will make threats that may or not be carried out, actions will be taken against the hijackers which may or may not result in the hijackers being killed or captured, and eventually it's likely everyone will live to go home to their families and put it all behind them.
All that changed on 9/11. We now know a successful hijacking is a death sentence. We hardened cockpit doors and, more importantly, we hardened our resolve not to comply with hijackers. I've only flown a hand full of times since 9/11, but every time I've boarded an aircraft since then I've decided that if something bad happens I'm getting involved. I've made the choice between me possibly being killed and everyone on the plane (including my wife) almost certainly being killed. I'm not going to sit there hoping there's an Air Marshall in first-class. I made that decision despite the TSA's insistence on removing from me anything that could outwardly be used as a weapon.
So we now have a new set of assumptions. We scan shoes. We frisk passengers. We produce pornographic images of teenagers. We allow children, CHILDREN, to be groped by strangers in a manner that would get them lynched if they weren't wearing badges (because apparently no pedophile would even THINK of applying for a job with the TSA). We can't bring nail-clippers, pen-knives, or bottles of liquid on board. All this for a false sense of security.
The terrorists are watching all this. They'll find the hole, the crease, the seam they can exploit. They'll make their plans and carry out their attack. If their attack is unsuccessful it won't be the TSA that'll stop it, it'll either be vigilant and courageous passengers on the plane, or it'll be plain old dumb luck.