You're missing my point a bit - it's not just about collisions, that was an example of one of many areas that 3D in Construct would become a headache.
As I said before, 3D-graphics-with-2D-gameplay is easy enough, and would be a good direction to move in. Construct as a full 3D game creator is what I'm arguing against. That means you need to start thinking about 3D angles, 3D trigonometry, 3D layout editors, 3D behaviors etc. etc. which IMO would be hard work to pull off.