"The fact is that it's impossible to get a gun onboard an aircraft."
Well, having trained with the guys in the Air Marshall service, having one of the instructors from my school being one of thier Chief instructors, and being an X ATC myself, I'll have to disagree with your expert opinion on this.
Do you have ANY idea how untrue and disproven your above statement is? Even discounting the human element of things, such as, oh, say, a member of an extremist group getting a 10$ an hour job as a security officer, or somebody bribing one, sneaking a weapon past security isn't rocket science, certainly a long way from impossible, and security exercises have regularly proven that the first line of defense breaks down rather easily. This absolute security you refer to is far from absolute.
Alert passengers and crew won't matter for a second if multiple threats armed with weapons get anywhere near the cabin area. The "re-enforced doors"...ya, I'm not going to talk about anything regarding this on the internet, but the guys here who are pilots will understand why. The point is, the pilot is responsible for the safety of his aircraft, the passengers, and now the potential targets his aircraft can hit on the ground. A weapon loaded with frangible ammunition is nothing to be afraid of, the pilots aren't just issued them without any formal training. It's no different than having a Marshall on board, other than that the pilot is ALWAYS there, the Marshall is not (of course, not all pilots are carrying weapons, watch that change if there is another hi-jacking someday).
People with your attitude Mora have been lulled into a place of false security. Go back to the Sept 12 or 13 2001, remember how you and we all felt, and see if your opinion makes sense.