All of it.

I don't think Bernie's a generationally good person (I think he's above average for a politician but not particularly unusual--and note that I think that politicians are mostly above average for the population). I don't have more or less faith in him than I would in Warren, Buttigieg, Booker, etc. Not a fan of Corbyn.
I don't expect you to adopt my moralistic appraisals of candidates, but I think there's very good basis to show that, besides merely being ideologically or politically rigid on major issues and principles, Sanders is exceptionally responsive, evolving, and transparent on smaller ones according to those major principles and issues. With almost all of the other candidates, you have massive moral (according to left-wing viewpoints) failings that they have been unwilling or incapable of defending on any political or moral basis, or even
explaining to left-wing critics, while Sanders has repeatedly opened himself to left wing publications to explain his past thinking on things like the Clinton criminal justice bill. With other Democrats, viewing them as moral equals to Sanders requires
a lot of benefit of the doubt with regard to their political records and their relationships with powerful private donors. We have to blindly
presume that votes in favor of deregulation are pragmatic good faith assertions on what will help the country, votes in favor of hedging key industries and corporations against citizen actions are pragmatic good faith assertions on what is just, votes in favor of military expenditures and authorizations are pragmatic good faith assertions that the former will fund and the latter will expedite the keeping of peace and the betterment of mankind, etc. etc. etc., because, ultimately, centrist liberals will never be held accountable for voting across the aisle on non-sexy issues.
For instance, and I know you're probably tired of hearing me insist upon this as a highly representative issue, the anti-BDS bill votes. Booker, Klobuchar, Gillibrand, etc. have completely dodged very simple questions on those bills' fundamental hostility toward the First Amendment, not to mention the political and ideological sovereignty of the country, and about their. Likewise, those politicians (Klobuchar and Booker stick out most blatantly given (a) Klobuchar's state, and (b) Cory Booker anti-bigotry platform) have repeatedly stuck their neck out for what's right when defending members of the Democratic Party - I'm thinking Keith Ellison and Ilhan Omar specifically - from dishonest and opportunistic Islamophobic attacks from Republicans or for indigenous activists during the NoDAPL protests.
I think what you're calling the "center-left" there applies to the MSM and many conservative pundits. I don't think it's an accurate description of the thinking of people called "center-left" in most contexts. Certainly not an accurate description of Chait's thinking. I also get the sense that you think Chait is a Biden supporter, and I don't think he is.
No, if I were a betting man I would be he votes Biden over Sanders, but I have no preoccupations about his candidate support.