I think Corbyn will perform better against Cameron, and any of the main contenders for the throne after Cameron steps down, than most people expect, especially at PMQs.
Furthermore his PQE policy idea is just an idea, an idea that has attracted the support of 41 economists, one of which use to work at the bank of england, and saw warning signs before the crash and was ignored, and another one is a noble prize winning economist, but nevertheless just a policy suggestion, and one he suggested as a possible alternative to borrowing to fund growth in infrastructure. It has also attracted around 50 saying it'd be awful, but, and correct me if I am wrong, I don't see a noble prize winner for economics among them.
But we do have to remember a lot can change in a couple months, let alone five years, the policies being suggested now by all of the candidates could be redundant by the time the manifesto is launched in 2020.