Im not a pilot but I would rather hope that when an auto-pilot disengages, it would leave things as they are rather than releasing the thrust!! You cant just let a plane stall because the auto pilot disengages. At the altitude these planes fly at, there isnt much leeway between stall speed and "bits of plane falling off" speed so you REALLY want to know what speed you are travelling at relative to the air around you and dont want that changing without a very good reason.
Given there were storms in the area I guess airspeed would have been jumping all over the place as well as altitude. If the computers couldnt fly it, Im pretty sure the pilots couldnt either because even in clear blue sky you cant tell your air speed from looking out the window.
My understanding is that the pitot tubes icing up is a very bad thing and if you cant get down to warmer air to de-ice them, youre in trouble.
Furthermore, I dont think anyone knows for sure if the messages received from the plane are the cause of the problem or a result of a completely different problem.
I hope they find those black boxes though and they are still in a fit state to clarify what happened to this plane.