Corbyn will NEVER lead the Labour Party to a General Election victory.
Why not? Well, start by setting aside all the misdirection about "principle", and "decency" and so on. And then put aside all policy-oriented argument (so beloved in the Westminster Bubble and among constituency activists). The rank-and-file electorate are genuinely too stupid and/or underinformed to understand nuanced policy arguments. I virtually guarantee that if you went to a large shopping mall in any city in the UK tomorrow and asked voter-age people there about detailed policy commitments of any of the major parties (let alone the differences between Corbynistas and Blairites within Labour...) you'd get a heady mixture of ignorant and incorrect.
No. There are two reasons why Corbyn will never lead Labour to GE victory. The first is that the vast majority of Labour MPs actively hate him (and the feeling is mutual). I know a few people within the Westminster bubble who tell me that there is a genuinely toxic environment within the PLP, where covert surveillance is taking place on both sides, and where virtually everyone is constantly looking over his/her shoulder. There is therefore zero chance that Corbyn's victory today will be the end of the matter in Westminster. Expect one of two things to almost certainly happen: either there will be another attempted coup (which will either succeed, or which will potentially tear the PLP totally apart in and of itself), or there will be a breakaway party consisting of the hundred or so Labour MPs who passionately hate Corbyn and McDonnell (which will probably trigger either a hundred or so by-elections or an early General Election).
And the second main reason why Corbyn will never lead Labour to GE victory is a much simpler one: he does not possess the visual or rhetorical attributes of a major international statesman. Do not underestimate this factor, especially with modern electorates who rarely watch the news and who are woefully under-informed on the issues. However "nice" and "decent" and "principled" Corbyn might (or might not) be, he simply is not someone who looks or acts like a leader of an important nation (in the eyes of voters).
There will, of course, be a cadre of life-long Labour voters who would vote Labour with practically anyone as party leader. All modern elections are about swing votes in key swing constituencies - that tiny minority of the voter population in a mere four-dozen or so (out of, currently, 650) constituencies who regularly switch their votes from party to party in general elections. And a significant proportion of those people simply won't cast their vote for Labour because they simply cannot envisage Corbyn as Prime Minister. Again, set aside all the complex and nuanced analysis of political issues and dogma etc. There are, in reality (for better or worse) a tiny relative number of people - probably not many more than 100,000 out of an electorate of some 40 million - who decide the outcome of UK General Elections. And those people (in part, by the very virtue of them being voters who, by definition, change their minds often) simply won't vote for a Labour party led by Corbyn in sufficient numbers to supply a Labour majority in the Commons.
(NB: all of this in in the context of the present constituency definitions and voter spreads. Come the 2020 GE, it is highly likely that boundary changes will have come into force, and the net outcome of those changes is likely to have a 20-30 MP relative reduction of Labour MPs (all else being equal). So, in all probability, Labour will require an enormous swing in order to gain a majority - this (IMO) simple will not be possible under Corbyn - for the reasons given above. It may not actually be possible for Labour under any leader in 2020, but that's another matter.....)
