I think focusing on Brexit discounts the fact that there was a concerted media campaign to demonise Corbyn. It was so intense I have never seen anything like that in western media. Even the coverage of Bernie Sanders does not come close in the sheer amount of brazen lies and slander being thrown his way. I feel even if he had the optimal take on Brexit which I agree he didn't he would have still lost.

Edit: I recall the press saying he was anti-semetic for pronouncing Epstein as "epshtein". It was nuts.