I think there's some good points here.Labour did remarkably well in the 2017 election. Weren't they 17 points behind the Tories at the start of campaign? If they'd had a week longer they might well have gained enough further seats to have been able to form a coalition with the SNP.
I don't know what shape the Tories are going to be in by the time of the next election. Only three of their MPs have formally left so far, but once Brexit is settled, as I assume it will be, there will be a lot of vicious recriminations from Brexiteers claiming they've been betrayed, leading most probably to further splits. Their overall performance over the past two years doesn't offer much of a recommendation for another five years in government. (That's not to say that Labour would have managed Brexit much better - I bet Corbyn and co are really glad they didn't have to try.)
In addition, I think there will be many voters who want "change", and many younger voters will have no memory of a Labour government and little idea of a what a left-wing government might be like.
So I wouldn't be at all confident in thinking that "such a party will never win a General Election". I fear that it might.
One of the traps May fell into in 2017 was complacency. Not so many years ago, we could have been discussing whether the Conservatives were *ever* going to win an election ever again, yet in 2017 victory seemed to be taken for granted. In 2010 the Conservatives only won with the help of the Liberal Democrats, and in 2015 it was still only a narrow majority compared to any of the three previous Labour majorities. The difference was that Cameron fought a decent campaign and offered an attractive manifesto, whereas May did neither - in fact she appears to have actively turned off voters with things like the social care "proposals".
I do fear there are a lot of rather naïve young voters who don't realise what a Labour government would be like under Corbyn and co.