Cargo Bay Design
Exhibit A was as you describe, made up of all the loads that the partner Nations wished to carry in A400M at that time.
Thus, it was that:
The Warrior Recovery Variant - weight 31.5 tonnes - drove the payload.
A German ground to air missile system - drove the cargo bay height
Vehicles side by side drove the width and so on.
Having then set the dimensions there comes a moment when the design parameters have to be frozen to allow design work to begin. These then drive the design through things like aircraft component loads and very quickly it gets to a stage where nothing basic can be changed. Adding 50cms to the width of the fuselage has huge ramifications for weight, drag and then power required and speed as well as changing the interfaces with the wings especially. Likewise increasing the weight has huge ramifications for the loads on the wing and the design has to change to reflect these which in turn means further weight increases.
There always is a danger that the world will move on and the only way to compensate is to design to over-egged requirements but this then has a knock on effect on cost. The bigger the aircraft the higher the cost. I saw all this during my years with Airbus and even now the customers are still asking for apparently small changes without consideration for what they in fact mean when translated into reality.
I remember years ago seeing the then Army Multi-Role vehicle platform (I cannot now remember the name) - it was specified to be air-transportable in A400M but was outgrowing even before any metal had been cut and that was before the mega amounts of armoured protection for Afghanistan.