Why wouldn't it work?
The outer case of the new Mac pro is round but the interior is a triangle.
It could be done and the logic board on the mini is quite small.
The design of the new MacPro will work because it has plenty of vertical space to arrange its key components, well, vertically.
This poor Mini mockup, doesn't. So key parts/main board would still have to lay flat as they do now. Hence it's cooling requirements are completely different, making the big fan on top of this mockup ridiculously oversized and would still be less efficient than the smaller fan of the older Mini's.
If a new Mini were to look anything like the new MacPro, it would likely have to be scaled down proportionally in all directions, instead of only vertically for the cooling tower design to work.
But a Mini doesn't really have the hefty cooling requirements of a Macpro, possibly making the design redundant right there. Plus the single main board of the Mini may have to be split into multiple boards, making it more expensive.
People expect a MacPro to be expensive. Not so much for a Mini.