That's probably true for the initial pitch down when you intentionally let the speed to increase to VMO with idle thrust but soon the ROD will decrease and you certainly won't lose 36000 in 6 minutes that way.
On what ground are experts still thinking in terms of a normal descent from 36.000 feet down to the ocean?
There must have been a upset scenario pretty much from the beginning to loose that amount of height in the probable time frame. In itself its nothing more then loosing the potential energy. Itīs traded for speed and or distance in the normal flight regime, and its used up by a lot of drag and loss of altitude in a deep stall scenario nd airspeed below flying speed.
To regain the speed, the drag has to be reduced by flight control input, normaly the stabilator to reduce the AOA. For that you need a favorable CG and full stabilator effectiveness (no out of trim situation like full nose up trim).
That simplified basic probably applies to most aircraft designs. Canīt think of AB being totally different when it comes back to basic aerodynamics.
I think it is pretty sure that an upset occurred leading to low speed, high sink rate and that continued to prevail until the final end. Aircraft had been lost before for that reason and a descent rate of 6.000 feet / minute does nor look unbelievable in such a situation.
Do we know anything about the position of stabilator trim and the position of the CG? Couldīnt find it in the amount of posts.