Malaysian authorities have issued new security instructions ordering that the pilot and co-pilot are not allowed to be left alone in the cockpit, even when one of them is taking a toilet break. A cabin crew member has to be in the cockpit until the pilot or co-pilot returns from the restroom.
So how many FAs know enough about the workings of the plane to know if the pilot is doing something fishy - much less know how to fly the plane in an emergency? For that matter, how many are burly enough to fight off an intruder?