Those that think having a member of cabin crew in the cockpit to replace one of the pilots taking a toilet break will make no difference as he/she could be easily overpowered are missing the point.
IF and that's a big IF the F/O committed this act it would have been far easier to do it once he was alone in the cockpit with a locked door behind him.
Having to overpower the member of crew sitting next to him would have been a deterrent just as it was a deterrent having a pilot sitting next to him....until of course that seat was empty.