The crucial thing that allowed September 11th to take place was that people on the plane weren't tuned in to the possibility of hijackers who intended to use the planes as bombs. If people had been thinking in those terms there is no real possibility that the hijackers would have been able to do it.

Armed guards on planes makes sense, but they'd be there essentially to minimise the number of casualties who would be caused in overcoming the hijackers. And they will need to be well trained and carefully recruited guards, and making sure that is the case will not be easy or cheap.

The main danger is still from explosives, and the thing here is that, when bombers are prepared to blow themselves up, rather than just set things in motion and watch what happens from safety, it makes it a lot more complicated and harder to stop them.