The thesis is an intentional act by a well informed person/persons in sole control of the flight deck. The Egypt air accident of the US east coast would be a similar category event.
I'm slowly beginning to think the same.
Previous pilot suicides involving a simple nose dive have always been found out and published, invalidating any insurance claims the suicide was supposed to cause.
So, perhaps, just perhaps, the desperate person in this case tried to be cunning and take the plane somewhere it'll never be found, thus ensuring any insurance they took out pays up?
Just as valid a theory as all the others!