Popular with whom?
I agree that he was popular within the party membership, but would disagree that Corbyn was ever popular with most of the electorate. Probably due to the media, but the unfairness in that is a fact of political life today. Personally, I always thought there was less to him than meets the eye.
I agree that Starmer's recent improvement in popularity is because of Johnson. He could indeed end up as the man who wins because he isn't someone else. I'll take that if it gets rid of the Tories.
I'll humour you... more people voted for Corbyn in 2017 than any other Labour leader this century.
Number of votes @UKLabour has won at general elections held this century:
1. Corbyn 2017: 12,878,460 (40%)
2. Blair 2001: 10,724,953 (40.7%)
3. Corbyn 2019: 10,269,051 (32.1%)
4. Blair 2005: 9,552,436 (35.2%)
5. Miliband 2015: 9,347,273 (30.4%)
6. Brown 2010: 8,609,527 (29%)
But don't let that cloud your judgement.