Hopefully soon enough people will have thought this 'F/A on the flight deck' thing through properly, and we can stop discussing it.
On any given flight there are considerably more F/As than pilots. Those F/As have undergone less rigorous screening and ongoing examination (no disrespect to F/As, that's just how it is). Therefore, there's a higher chance of getting a psychotic F/A on a flight than a psychotic pilot. So I don't see how we solve the Germanwings scenario by shutting an F/A into the flight deck with crash axes, extinguishers, oxy bottles and an unsuspecting pilot, when statistically it seems more likely for the F/A to be a problem than the pilot.