Wino,
I understand your point, but I think that even if the door had remained locked (are we sure at this stage that it did not, by the way?), it would not have prevented this disaster. Smashing a flight deck door looks real easy, and for a real coward, taking a stewardess hostage to get someone to open it is probably easier still.
On a more positive note, I agree that strong pax involvement is likely to happen more often as a result of all this.