I have followed this thread from the outset,but have not contributed because I didn't feel that I had anything worthwhile to add. All i will say is this. The number of commercial airline pilots employed throughout the world probably numbers tens of thousands,the chances of none of them going sufficiently mad to commit an act such as this are probably quite long.
You can't really get over the problem by installing sky marshalls because the same mathematical formula applies,and I would have thought it quite likely that in general people losing it are going to try to cover their deteriorating mental state.
Interestingly we have not seen it happen with coach drivers who could achieve something of a similarly horrific outcome on an Alpine road. The Moorgate tube disaster is one where the possibility of driver suicide remains not ruled out.
Is it the spectacular nature of an air crash that is the main motivatiion? Sadly I think what has happened makes a re-occurence more not less likely and I cannot imagine any scenario that could entirely prevent it if an individual is minded to try. Only time will tell of course if it does happen again.
An inevitable corollary will be a big loss of passenger confidence in airline safety. Mechanical and technical failures have been largely eliminated and we are now faced with human frailty which I think may prove a much tougher nut to crack.