It depends what you mean by ‘credible alternative’, surely it means a Labour Party which appeals to enough voters to get elected (and yes that will include sometime tory voters). I reject the assumption that Labour can only get into government by either out torying the tories or by embracing Corbynism. I don't think the majority of voters want either.
I’m not ‘terrified about the prospect of Corbyn as prime minister’ at all, why would I be? Crobyn’s politics aren’t very different to mine and I would be delighted if Labour won in 2020 with him as leader. What ‘terrifies’ me more than anything is the tories winning again and being in government until 2025.
You are right about evidence and it is difficult to prove that JC won’t win Labour the next election, although given his poll ratings I suggest it would be even harder to make the case that he will. I’m afraid that 60% among Labour members isn’t going to translate into anything near the 35%-40% of voters in total needed to win even a narrow majority.
Also, along with the electorate’s obvious distrust of him over the economy, probably the biggest obstacle Labour have to winning in 2020 is Corbyn’s opposition to nuclear weapons. Unfortunately the British people are wedded to the idea of an independent deterrent and imo they’ll never vote to scrap it. There’s also the question of his age, he’ll have turned 71 by the next election and 76 by the end of his first term. It shouldn’t matter, but who can honestly say that it won’t to a significant number of voters?
The issue for me is still how to defeat the tories in 2020 and unless anyone can convince me otherwise, I can’t see Labour having a cat-in-hells with JC.