The "this defeat was all about Corbyn" take has to explain why we got the very different results we did (in strongly pro-Brexit seats especially) two and a half years ago when we *also had him as leader*.
Now, it is quite likely he became (even) more electorally toxic in the intervening period. But given how differently "leave" areas performed in this election to "remain" ones, I can't help feeling this sort of ignores Occam's Razor. In 2017 we promised to respect the referendum result, this time we did not.
(or at least were overwhelmingly perceived as not doing so by those who voted for Brexit)
This enabled Johnson to run the sort of campaign that May had wanted to, but couldn't, last time.
Theresa may was a bad campaigner and before 2017 the ratings for both leaders converged from +30 fir Theresa to +5. And she was a bad campaigner and seemed too posh for a lot of people. Boris on the other hand had a approval rate that was 20-30% higher than corbyns. Also people dont realize in this thread how much having a bombastic personality like trump and bolsonaro, made a lot of working class who voted left wing to switch.