Evidence that PPAD is hard? 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?
PPAD is pretty "low" above P and not much would change in our understanding of complexity if it was shown equal to P (except that the few problems in PPAD would now be in P).  The main "evidence" that PPAD!=P is an oracle separation, which is essentially equivalent to the  combinatorial fact that no "black-box simulation" exists.