Dozy;
In the small amount of research I did a while back I had the impression that so long as the airplane wasn't in Direct Law, and the auto-trim system was functioning normally, one could set trim using the wheel because the system senses a manual input and releases the brake on the hydraulic motor driving the THS. Once that motion was no longer sensed, to me it was logical that the system would return to normal and try to satisfy whatever orders were being received by other systems. Some have said that order is the 1g order and so we may have seen a return to the -13deg setting, or, if the stick had been full-forward for sufficient time, it would have returned enough to provide elevator authority sufficient to reduce the AoA and unstall the wing.
All theory of course...
In direct law we know it is a different story; that's why the red ECAM warning.
The design seems to boil down to the engineers imagining and expecting this scenario. Hindsight being what it is, we can now say something but I'll bet anyone here that no one imagined such a scenario to the point where the design would have been changed - certainly not the certification people.
Just my personal opinion but to me that is entirely crystal ball territory. One of course cannot imagine and account for everything and still get the machine off the ground.