Everyone knows that Corbyn ultimately failed, so surely it's the wrong benchmark. And as you point out, Blair is the only one who has won, so why is that not the relevant bar?
The '20 points ahead thing' was started by Blair. It was a daft thing to say as (even) he was never 20 points ahead.
Starmer was given the benefit of the doubt. That was why he won. 40% of the people who voted for Corbyn both times voted for him. It's his performance over the last year which has turned a signifcant proportion of those people against him.
Let's hope the criticism of the last couple of weeks has stung him into upping his game. I hope today's speech goes down well.