I know a retired airline pilot who is totally against the crew being armed. He said the pilot and co-pilot can lock the door and the flight attendants know they are expendable. This is from a sharp guy who used to fly F-4;s off a carrier.
This is true, the doors now can be locked securely. The problem comes when a pilot needs to go to the restroom. Or on a long flight wants something to eat. Restrooms and galleys are not in the cockpit.