Well, let's say it's the heist of the century…. where's the plane?
The hypothetical gold might be expensive, but not really when compared with over 200 lives, or the price of a replacement for the aircraft. How much would it cost to arrange the level of complexity in a plot to steal the gold while airborne, have it land somewhere safe for you to offload, and then safely disappear with it.
The lack of people on their delayed way to Beijing would seem to argue against a robbery.