The calculus is thus:
The Left is already highly motivated to turn out this election, our disdain for the current occupant of the White House demands it.
In and of itself, our motivation to vote may not be enough to overcome institutional roadblocks and secure the office of POTUS.
There are many on The Right who are un-enthusiastic about Trump, and may be inclined to vote against him (or not vote for the POTUS at all) if the candidate we run does not trigger the knee-jerk "OMG SOCIALISTS!!!!" as readily as some of our more left leaning options.
In order to win, it is likely that we will need a candidate who will do well amongst the (small) number of Independents- while not stirring the lukewarm to cold Right leaning voters into action.
At this time, Biden seems to be the best prospect for that. I don't think many of us actually expect him to govern (or even survive his first term), but he may be the one most able to give us a "rubber stamp" presidency.