1) Even in southern England, there is one factor that is a far bigger voter-repellent than a radical left party leader - and that's disunity. According to YouGov, Labour went from enjoying a 3% lead in late April to being 11% behind by mid-July. Corbyn was leader in both the 'before' and 'after' photos, so self-evidently he's unlikely to be responsible for the slump. The only things that have changed since April are that Theresa May is enjoying a honeymoon period, and the right-wing of the Labour party have needlessly started a civil war.
2) Some of the plotters claim that they have no problem with Corbyn's policies, but only with the man himself. If that's genuinely the case, an obvious deal was available that probably would have been acceptable to all sides - a change in the leadership nomination rules to allow Corbyn to step down (perhaps in a year or two) safe in the knowledge that a more charismatic alternative left candidate like John McDonnell or Clive Lewis would have no difficulty getting on the ballot paper, and would probably become leader. The fact that none of the plotters seem to have even seriously contemplated that possibility tells you all you need to know - this isn't about electability, it's a destructive ideological crusade against the radical left.
3) For all the mythology surrounding the 1983 election, the Labour right actually have a much longer and more ignominious track record of losing elections than the radical left. It was the right, not the left, that lost the 2010 and 2015 elections. The right were also responsible for the landmark defeats in 1979 and 1992, and arguably also in 1987 (Neil Kinnock was midway on his journey from the soft left to the right by then, although admittedly he was still nominally in favour of unilateral nuclear disarmament). And the right can't entirely escape responsibility for the 1983 calamity either - it seems undeniable that the Tory majority would have been much more modest had the SDP split not taken place.
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It takes a strong Westminster Bubble angle before Mike "can't be arsed" Smithson will pay the slightest attention to a Scottish poll, and sure enough he's beside himself with excitement about a new full-scale Scottish poll from YouGov showing that Jeremy Corbyn has worse personal ratings among people who voted Labour in 2015 than among the electorate at large. But for my money, this poll tells us far more about the character of Labour's rump support in Scotland than it does about Jeremy Corbyn's prospects. Corbyn actually has comparatively good ratings among people who voted SNP in 2015, which is perfectly logical, because that group contains many traditional Labour voters who had switched to the SNP for the first time ever. It's clear that Corbyn's leadership hasn't been sufficient to bring them back into the fold, but the idea that they're more likely to be won over by a New Labour-type leader is fanciful in the extreme.
It's also telling that Ruth Davidson has much, much better personal ratings among 2015 Labour voters than Nicola Sturgeon does, in spite of Sturgeon's impeccable social democratic credentials, and the fact that she's more popular than Davidson among the general population. That makes no sense at all unless the legacy of "Jackanory" Jim Murphy's leadership has been to transform Labour's support into a narrowly-based, centre-right, hard-line unionist sect. (I hesitate to use the word 'cult', but...)
Theresa May's showing in this poll is being portrayed as reasonably good, but you have to be very cautious about polls conducted during a political honeymoon. For example, an early Scottish poll after John Major replaced Margaret Thatcher showed Tory support back above 30% for the first time in many years, leading Bernard Ingham to obnoxiously claim that everything was back to normal and that the only reason for the Scottish Tories' travails had been the sexism of voters north of the border (including female voters, presumably).
You don't have to look far to spot the warning signs for May - even at the height of her honeymoon, more Scottish voters say she is dislikeable than say she is likeable. Perhaps the most significant finding is that 54% of respondents say she will either be a worse Prime Minister than Cameron, or no better. When you bear in mind how awful Cameron's personal ratings in Scotland have been, that may be a clue as to where she is headed in the not-too-distant future.