Gordon Brown is an impressive speaker (if he'd learn to stand still – I'm still dizzy) but the narrative that most people who support Jeremy Corbyn are not interested in whether he is electable is silly. If I didn't think Corbyn could be elected I wouldn't be supporting him.
Brown lost. Miliband lost. Cooper would lose. Kendall would lose. Burnham would probably lose.
The reality is that Corbyn has the best chance of winning in 2020 if the party unites behind him.
First, Corbyn can expand and energise the party to make it a real campaigning force, a real social movement that can make its case effectively to liberals and centrist Tories. Second, he has the best chance of winning back those 40 seats in Scotland. Third, his straight-talking–but polite–authenticity can win over those who don't vote, vote UKIP or don't care because they believe all politicians are all the same.