I think this is wrong, I think he can bounce back, or at least Johnson can nose dive.
Johnson has three years of difficult government ahead of him, and he's got different wings of both the party, and the Tory electorate, pulling him in different directions.
I am absolutely not saying that I think Starmer will win in 2024 with 400 seats, I think he'll probably cruise to a lesser defeat than in 2019, with the LD's making gains in the Tory south, and the Tories losing some of their northern gains simply because of indifference amongst the brexity/historically-Labour electorate.
I accept that a change in leadership 3 years before a GE raises the possibility of a better result, but I don't accept that it's a probability, and I think it carries a far greater risk of danger than it carries a chance of success.