The unanswerable question is how many atrocities have been prevented by the raft of security measures since 911 (and others such as baggage screening following bombs in earlier years). Airlines (especially flag carriers) are highly lucrative targets for terrorists as they strike fear into the populace and are seen as a national totemic symbol of the nation you are trying to attack/alter the governments courses of action. So it is a matter of balance - in my view cockpit doors have probably saved more lives that they have ended. Nothing in the above statement detracts from the absolute tragedy here.
I also suspect that whatever the motivations of this individual, he would have found another way of disabling his captain with or without a cockpit door mechanism.
On a different tack, there is an irony between the number of posters resisting the placement of in cockpit cameras for privacy reasons and the number calling for the early release of the pilot's names........