The idea that they do is based on Atlas nerds assuming all voters think like they do. I laugh at the idea Tulsi Gabbard automatically inherits all 13 million+ Sanders votes if she runs for this reason (and of course the notion that "Bernie Bros" made up the majority of Sanders supporters and not just some neckbeards on Reddit) and that working class white Democrats want to punish Sherrod Brown for endorsing Hillary or whatever. Most voters don't even KNOW who endorsed someone four years ago.
I'd go even farther than that, and note that the "Sanders coalition" is heavily dependent on college kids, and most of the college kids of 2020 are still in high school now, and probably paying little attention to politics. Whatever ~10 million+ votes the next Sanders-like candidate gets will largely come from voters who are currently too young to vote (slight exaggeration).
