Point defense such as you are describing is almost impossible to successfully implement, and can easily be overwhelmed. The Israelis do it, but they have no illusions that it will stop a determined enemy.
Their philosphy is one of concentric defense, with defense at the point of attack being the last resort. They much prefer eliminating the threat, early and as far away from the terrorists’ target as possible.
They go after the planners, the financiers, the recruiters, the indoctrinators, the keepers, the inspirational and operational infrastructure of the terrorist organization whenever possible. To try and stop them once they’ve reached the target is to fail 90% of the time.
A (retired) Mossad agent told a group of us once, “You have people who need killing, and you must kill them. You Americans have not yet learned this lesson. But unfortunately, you will.”
Actually, our government maybe has learned the lesson. The drone strikes that Obeyme are so fond of are actually a good start (and the only thing on which I can agree with him).
But it’s like hamburgers. Everyone (’cept vegans) likes the product, but they are horrified if shown the process (’cept farmers). Call it hypocrisy, whatever, but people are capable of VERY selective perception.
That’s why the TSA and our entire approach is so bad: it is security theater, not real security.