I have a theory.
http://www.bbc.co.uk/newsbeat/article/34232177/jeremy-corbyn-young-people-have-to-be-heard
I do not believe that Corbyn is expecting to win the next election. I think he is playing a longer game, planting a seed that may grow.
He is looking to empower those that he can, reach out to those he has always sought to help and establish a young empowered and active socialist base to continue his ideology and drive Labour in the future.
He will not compromise his principles to do something as vulgar as win popular favour, indeed, he is no idiot and will be aware that in the current political climate he couldn't possibly win an election, He will however have belief that a longer term revolution, beginning now can spring from his leadership. It plays to his strengths as an idealist, as a lifelong martyr to his own principles. It also explains why he is so willing to go against political convention with the media and things such as PMQ's. He doesn't care for the frills, his endgame vision is that of a socialist Britain, embraced by the young all buying into his ideals who make it happen later.