<...With enough people pushing, that might be an advantage unless the hijackers got a gun from the cockpit. Then they just shoot people and let them stack up.

Your thesis, as you make clear throughout the post, is that the first thing that's going to happen in a hijacking is that for some arcane reason, one of the cockpit crew is going to unlock the door and throw a gun to the hijackers.

Failing that, if you can come up with some plausible way for a hijacker to bust in a door barely one person wide with a crewman aiming a loaded weapon at the opening, and then take the gun away from him, you let me know. Unless the first hijacker through the door is Clark Kent.

Anyone with common sense would realize that an armed cockpit crew has a much greater chance of killing hijackers than unarmed hijackers have of somehow swiping the guns. I do know that an unarmed crew has much less chance of success of dealing with the hijackers at all.

Your scenario is at least more plausible when discussing an armed air marshall in the passenger cabin, although they would still have to get the jump on him. And I don't think anyone is seriously discussing arming the stewardesses (which given how irritating some passengers can get, is probably a bad idea anyway).