I think you're making too many assumptions. You are assuming that the terrorist dickweed can crash a jetliner at will WITHOUT getting to the cockpit. One of the only sane, logical changes post-9/11 was to reinforce the shit out of the cockpit door, and put procedures in place where anytime that door would be opened (pilots going to the toilet, etc) there are obstacles in place that would allow time for the door to be closed quickly. In short, the easiest way to get to the cockpit would be to start taking hostages using deadly force. The passengers, knowing that they are going to die anyway, will quickly take down that threat (see personal injury vs. complete annihilation argument above). So where would the entrance of a firearm play into the passenger's favor? Assume that there are multiple dickweeds, one of them being behind the armed passenger. Now the terrorist has a firearm, and the entire balance of power changes drastically.