You are still overlooking the obvious that jb747 pointed out. That is that a pilot can, if he wishes, make the a/c go out of control very quickly. In this case for example all he had to do IF they were actually gaining entry into the coughpit is increase the rate of descent and hit the ground a whole lot quicker.
There are many ways to quickly disable an aircraft. This is a statement of fact NOT of opinion.
Yes but... in this case the co-pilot didn't do that. He waited till he was alone, cut off from the rest of the plane, and then acted. If he was going to crash the plane regardless, he could have done so at any other time, with the other pilot there. But he didn't.
Maybe he didn't have the guts to do it with someone else there. Maybe having someone else there made the human element too great for him to go through with it. Maybe if the pilot hadn't gone for a toilet break on this flight the plane would have made it safely.
Perhaps investigators will turn up evidence to say for certain 'this was the flight' he was going to crash. But if they don't, then no one will be able to say for certain that having someone else there wouldn't have prevented this incident.
On that basis, having a second person there at all times should logically provide reassurance not only to passengers, but to the other flight deck crew and cabin crew.
Sure any person or can probably carry out the
physical act of crashing a plane. But the
psychological element (particularly things that might work against a trigger) needs to be factored in as well.
Egyptair 990, Silkair 185, LAM 470 - in all cases the pilot responsible waited for the other member of the crew to leave the coughpit.