In 1999, before the advent of the current cockpit doors, an Egyption Airlines First Officer who was about to face disciplinary issues from his airline, managed to crash a 767 into the Atlantic Ocean and kill 217 people, even though the Captain had gotten back into the seat and was trying to recover the aircraft.
Having a FA sitting on the jumpseat will do FA to prevent a re-occurrence of this.
While it is possible now to quantify the number of lives lost as a result of increased cockpit security, and keen observers also class MH370 as a probable candidate here, it is not possible to put a number on how many potential lives have been saved by these measures.
The answer to this problem lies in monitoring our fellow pilots and taking an interest in their welfare. Aviation is a balance, a compromise. Always was, always will be.