Hot take: the rest of the party has nominally adopted some of his rhetoric and positions with little risk, widening the field and leaving only his core base invested.
If his goal was to push the Dems left, there's probably some success there but he's not and never was going to win.
Bernie and the people in his campaign certainly want to win and believe they can win.
After 2016 there was a pretty plausible case for Bernie's electability (something that's been a central part of the PR coming from his team), when you could say he won more than 40% of the vote while starting from no name recognition and having the entire formal party endorsing his opponent, his approval ratings were high, and he polled better against Trump than Hillary did. None of that's true now.
I think that's why you saw Harris, Booker, Gillibrand, etc. rush to cosponsor his M4A bill. There was a sense that his wing of the party was ascendant and you had to get on their good side to have a chance. I think that perceived clout's dissipated a bit.