The party has moved to the left since Bernie entered the primaries. Clinton tried to catch the wave, but had too much Goldman Sachs baggage. You can argue from the election data that she lost because she couldn't bring all the Bernie voters to her side.
If Jim Webb had stayed in the Senate, perhaps he would have performed better in the debates. As it was, he did poorly and left the impression of an irascible old man. It worked for Bernie because he had built a theme around his candidacy.
As volatile as the political atmosphere is today, imagine what it will be in 2020. At this point no one can say with certainty that Trump will be the Republican nominee. How can we possibly know who will lead the Dems?
Given those caveats, why not speculate? There is nothing to lose.