It's hard to gauge the influence of a losing politician without the luxury of history, but it's very possible that Bernie becomes a William Jennings Bryan-type figure to historians.

During the Gilded Age the Democratic and Republican Party weren't that different...

WJB got the nomination 3 times and lost 3 times, but the Democratic Party basically transformed into the progressive party in the 20th century because of him, while the Republicans continued to trend to pro-business conservatism.

Bernie didn't get the nomination either time...

Though party's today are much less forgiving of politicians that lose than 100+ years ago, but it's been genuinely surprising to see so many things that most politicians wouldn't even mention 5 years ago (M4A, free college, etc...) become central debates in the Democratic Party..

Though, who knows, he could also just become a footnote to history and the Democratic Party under Biden and his successors will solidify its base with upper middle class suburban white voters and cement itself as a liberal, centrist/center-left party...

I do have my fingers crossed for a random Scopes Monkey Trial-esque plot line to develop though (one of WJB's last public acts was prosecuting a teacher for teaching evolution, essentially for a publicity stunt).

(side note, I've been reading a lot about William Jennings Bryan)