Though, if he were a woman (or POC), a heart attack would have ended the Sanders campaign. And I don't think he should have gotten in the race. I think he should have just endorsed Warren once she got in the race.
Anyway, all that is beside the point. His support is different; it's cult-like. As with Trump, Sanders has a cult of personality. His following is substantial and loyal but it isn't going to grow much, if at all. Nor is it going to shrink much, if at all. No matter what he does or says. Whereas someone like Booker or Klobuchar doing well in Iowa could potentially propel them to the nomination (remember that Kerry was very low in the polls in late 2003), Sanders will essentially be done after New Hampshire (or at least Nevada).
Also, Sanders is the only candidate who ran in 2016 and the data we have from 2016 makes it clear that he won't be the nominee unless he can somehow do much, much, much better than last time among POC. I see zero reason to think that'll happen. You can't get your ass kicked among Black folks, in particular, and come anywhere close to winning the nomination. No way, no how. That's why we knew on Super Tuesday in 2016 that the race was over. A race that was tailor-made for Sanders, as he was *the* alternative to a polarizing frontrunner. He doesn't have that same advantage of a 1-on-1 race against a much-vilified opponent that he had in 2016, so the pressure on him to leave the race sooner will be stronger in 2020.