If they completely separated the cockpit from the cabin then you don't need the marshalls. I suppose it's too expensive to change a fleet design though.
I think the sky marshall thing is just a warning to would-be terrorists. Much more clever to play with the gasses (the pilot controls O2 levels apparently) - maybe with a little more finesse than the Russians did in the theatre seige though! Knock them all out and arrest at your leisure. Co-lateral damage would be less than a gunfight. However, unless this was made public knowledge, there is no deterrent effect and I doubt they'd want to make it public. Respirators would be harder to smuggle on board, and would be useless against low O2.
I like the "put it into a steep dive strategy" though! Sounds like fun. Can you loop a 747?
