There is often-quoted philosophical justification for believing that P != NP even without proof. Other complexity classes have evidence that they are distinct, because if not, there would be "surprising" consequences (like the collapse of the polynomial hierarchy).
My question is, what is the basis for belief that the class PPAD is intractable? If there was a polynomial time algorithm for finding Nash equilibria, would this imply anything about other complexity classes? Is there a heuristic argument for why it should be hard?