Like my barber said, a toothpick
can be threatening, if you're mean enough.
I find it incredible that they could hold people at knife-point. Maybe I'm not so easily intimidated because I'm bigger than most people I encounter. But that's the key: the crew and passangers should have more confidance!
Picture this: would-be hijacker uses a sharp implement on the captain. He plays along to buy time. Then the stewdess breaks his skull with the fire extinguisher.
No security guard with a dedicated job, but what if all the beverage servers also had martial-arts training? Maybe just one is a blackbelt, but everyone would have more confidance in a situation and could handle themselves in a manner other than as sheep.