By you claiming it a success because the polls had him at 20%+ and he got to 40% in the actual election, you must believe somewhat in the accuracy of the polls. What do you think of Labour being at 18% currently then? If the polls are accurate he’s lost over 50% or his support in 2 years.
The reason he got that high was because of May’s total abomination of a campaign. It was gaining votes through default and because Corbyn’s manifesto had a wish list that was undeliverable, especially if we’re going to be worse off because of Brexit. For example wiping off student debt of students that had already left, which he later went back on after the election.
If he loses again the next time are you willing to concede he’s just not up to it?