I accept all of this is true and the electoral system for the UK produces some misleading results.
The big problem for Corbyn was that he was such a polarising figure. Tony Blair won a majority not simply on the votes he got but because there were plenty of people who would never vote Labour but were not that bothered so they did not vote at all - or they voted Liberal Democrat which had the same effect. In 1997 there was not much of a Anyone-But-Blair vote. Obviously that changed later on. By contrast, there was a determined Anyone-But-Corbyn vote - not so much amongst Labour voters but amongst everyone else. For example, Liberal Democrat voters who couldn't imagine that Boris Johnson would be worse than Corbyn.
It is the same thing in the USA. Hopefully the Anyone-But-Biden vote will be less than the Anyone-But-Clinton vote.