A few thoughts about Bernie floating the idea in March 2016 of opposing Clinton's delegate lead at the convention through a super delegate pitch. First, as Warren recognized, Bernie was facing a profoundly undemocratic rigged process. She called it out here:
Second, people forget how nervous everyone was in March 2016. There was a palpable fear even among party elites that the FBI would announce a felony indictment against Clinton later that summer. So in March, the Sanders camp signaled they would stay in:
politico.com/story/2016/03/…
politico.com/story/2016/03/…
When Bernie's campaign proposed this idea, there was push-back immediately from progressives. People like @benwikler were telling Bernie that using the super-delegates was the wrong move. It's like using the ring in the Lord of the Rings. 
And as we know, Bernie backed off this idea and did not stay in until the convention, choosing instead to drop out that summer and begin working for Clinton.
There's also this idea that Bernie "approved" of the current super delegate system, which is not how that went down. Ask folks about the Unity Reform Commission, like @NomikiKonst, and they'll tell you how Sanders reps fought to end super delegates but didn't have the votes.
The party elites were able to keep their system mostly intact, but every good democratically-minded leader came out of 2016 hating the idea of super delegates, especially Elizabeth Warren:
The short story is that Bernie's camp had a bad idea in March 2016 when they pitched the idea of convincing super delegates to contest the convention, even in the face of an obviously rigged race. And it's an even worse idea being floated by the Warren camp now in 2020.
