Putting the cpu/gpu of the 16" MB Pro would enhance the mini greatly. The thermals would be be much better than in a laptop resulting in better performance. Yet, you wouldn't have to pay for an unused screen/battery. Also, while you can use the MP Pro in clamshell mode, it is making the thermals even worse. So why should one get a laptop, if one wants a compact desktop?
Apple's hardware business model is driving margins by bundling unnecessary features in the "trim levels". They don't want you picking and choosing only the parts you need.