Since the hijackers can't take over the plane, there would be no reason for them to cause trouble among the passengers. If they did, however, the crew would be able to flood the compartment with gas and land the plane at the nearest airport.
Well, I dunno. I wonder how long it would take the hijackers to figure out that they can get the oxygen masks down and clamp them on, or to carry little gas masks that look very much like duck calls. IF the plane is also upside down a couple of times during the trip to the nearest airport, that plan would work much better.
However, is this instead of allowing trained passengers to be armed? How many people would, then, die while the gas was flooding the passenger area?