I very much disagree that having 2 people on the flight deck would be useless. Not only is it a good practice to begin with, but also has value in prevention. It's one thing to run an airliner into a hill safely in front of a locked door with faceless entities behind you and quite another to have to brutally attack and incapacitate another person--up close and personal--who would be directly fighting back for their life and then go do your evil deed. During the while the other individual could be doing SOMETHING and might even manage to get the door open.
In fact, most of the mass attacks by crazies on groups have been done with the premise that they would not or could not fight back under the circumstances (disparity in armament and/or force and shock with a lack of understanding of the victims what was going on).
Now that this HAS happened, crew know that it CAN happen and think about what to do in such a scenario.
While it might not stop dedicated miscreants (like in the FedEx case which was a planned attack in a suicide for insurance situation), it might be enough to keep a crazy off the ledge long enough for other factors of behaviour to be discovered through peer monitoring and real time personal history factors.
Obviously this disturbed individual was looking for the opportunity to be alone to do his deed. While the 2-person concept isn't a cure all by any means, it is a start and I believe has value at little or no cost. Why not ?