Because your correct solution for NatPos : Nat+Pos in a language without first-class intersections and unions required you to use 4 traits of boilerplate; whereas, as I had already explained upthread that in my proposal with the first-class intersections I can write instead Nat /\ Pos and only 2 traits are needed to attain the same correct semantics and safety. I expounded on other reasons (my replies will be forthcoming at the linked thread).