[higherkinded]

>which makes n-arity functions necessarily first-class

Excuse me but tuples. You still have to observe them to evaluate them but that's all you need to simulate n-arity in the way you want.

>more dynamic

Well, Hindley—Milner is how you do type-safe stuff with inference that annotates for you. For any generic purposes you just put a typevar with constraints, so I don't see a problem here and can't really get what you gain. MLs are more about proper composition.