I thought this tweet thread was interesting as regards Warren:
Basically, Warren's candidacy was meant by some to help square the circle — for people who said they liked leftist policies but thought Bernie was bad on social issues, Warren could do both. (I think this telling underrates the elite personal animus against Bernie a bit, but Warren solves both.) Whether you personally think Bernie is bad on social issues or not, there's no denying a lot of people think it.
In this model, Warren doesn't need to win all of Bernie's supporters. She just needs to win some of them while also consolidating the Clinton voters who cared about both economic and social justice. And she did win those Clinton voters, if Poliera's love affair is any indication. The problem was that she failed to pull together elite support or moderate Dems, because it turned out it wasn't about Bernie being untrustworthy on race — they really just don't want the policies. Warren couldn't overcome that.
Honestly, the guy's twitter thread put a loooottt more thought into the who's and why's of Warren's drop than the people who just drifted away from her did. Warren's issue is that her support was soft and tuned out, not sold but not against, sorta just picking her to pick someone. There were polls on how loyal her support base was and she was 20 points behind Sanders/Biden in that regard, so a blow up was fully possible. All that was needed was a new flavor of the week to get on the radar, Buttigieg spent his money, he got his share. Bloomberg then lapped up at Buttigieg and here we are.
Her boost was mostly from people dropping out and kinda settling on her without thinking much about it, which was her strength. If people kept dropping out, if Buttigieg, for example, had dropped out, she would have had a real shot, but the opposite happened, torpedoing her.
I don't actually buy that it had much to do with her explaining M4A. That's the always online talk. Voter drift is usually a lot more basic than that.