The only reason I would buy Construct 3 would be if it were a 3D version of Construct 2 and had the possibility to make 3D games as easily as 2D games on construct 2. This would be amazing and worth the 100$ per year rent if not more.
I can buy Construct 2 once and use it for eternity for 2D games so renting Construct 3 would be pointless if I could only make 2D games.
EDIT: I might buy it anyways