I don’t think the progressive left is why Hillary lost. Certainly, people choosing not to vote because she was centrist had an impact, but I don’t think there were really enough “Bernie or bust” folks to be solely responsible. You actually had a higher percentage of Bernie voters going for Hillary in 2016 than you did Hilary voters going for Obama in 2008.
You had like five different factors and a pretty tight final margin. No one factor was responsible. Comey’s letter, Hilary being center, Russian meddling, online misinformation – all of it is partially but not wholly responsible.