Armed citizens are nice, but armed citizens are the last line of defense in war, not the first. Make no doubt about it, this is war. These guys want to destroy the country. In every war we've ever fought, even the Revolution, the citizens were the final line. And the problem with this war is that the front is whichever plane these guys try to take, and we won't know the war is even on until it's far too late.
This isn't the hijacking of the 70s, we're not talking about having a hundred people scared and inconvenienced and maybe 1 or 2 killed. We're talking about having buildings knocked down, thousands dead, economy F'd up for months. Hardcore no nonsense acts of war. If we were still dealing with 70s style hijackers you'd be right. But that time is gone. The new time is that we have hundreds of guided missiles in the air at all times and security, layered security that starts as far away from the plane as possible and ends in the cockpit, is how you keep them from going off.
When someone is he11-bent on crashing an airliner, armed citizens ARE the last line of defense in that war.