It is true that Sanders did very poorly with African-American voters in the South, which more than anything else cost him the primary. But that is not to say that these voters would not have voted for him against Trump had he won the nomination (or, sadly, that their support matters much beyond the primary, as the states that swung victory for Clinton have not voted Democrat nationally since the Civil Rights Act).
And on the other hand, Sanders is uniquely equipped to make the Party competitive in places where it has been (deservedly) routed since the turn to Third Way orthodoxy under Clinton. If he wins the nomination in 2020, do not be surprised if West Virginia becomes competitive. Seriously. People who get their news entirely from liberal twitter think of West Virginia as ground zero for Trump, and essentially populated by stone age mutants, but it was one of the most reliable Democrat states for decades prior to the Clinton turn. It was not so much WV abandoning the Democrats as the Democrats abandoning WV. Candidates who run on a progressive platform there are already looking very promising, against enormous odds:
https://theintercept.com/2019/05/18/west-virginia-populist-governor-campaign-stephen-noble-smith/ Joe Rogan comments and wildly successful Fox News appearances are obviously not scientific proof, but they hint at something which should not be so flippantly dismissed either.
At the very least, it is entirely plausible to suggest that with Sanders rather than Clinton, the Dems would have won Pennsylvania, Michigan, Wisconsin, Iowa, possibly even Missouri - and/or Ohio, and thus the election, likely by a comfortable distance.
And it is a serious misconception that Sanders performs poorly among minorities. The Politico link you gave is indistinguishable between him and Biden. And that Monmouth poll had him as the preferred candidate of what they call 'voters of colour' (while also positing that these voters' support for Biden had dropped by about half). Somehow (for some mysterious reason), the media continues to reinforce the impression that Sanders performs poorly among minorities, when it is actually one of his great assets. It is quite revealing, actually, to recall that the same Clinton hacks made the same already tired accusations of toxic masculinity against Obama in 2008:
https://www.salon.com/2008/04/14/obama_supporters/, which might prompt us to consider: are these claims actually true?
Where Sanders does perform relatively poorly is with older white wealthy well-educated professionals - which is the core support for Harris and even more so Warren and Buttigieg (who usually polls at 0% with African-Americans, which also mysteriously never gets mentioned). But, in a reversal of the assumption that it was the left that had nowhere else to go, it's hard to see voters like this, the most shrill anti-Trumpers, not begrudginly turning out for Sanders should he win - and in any case, they mostly live in states that the Dems will carry regardless. Would it make sense to run with the candidate who can deliver minorities, white professionals, the disaffected post-industrial working class, and overlooked but most importantly, the huge bloc of people who otherwise don't vote - surely the only hope of taking back the Senate and actually implementing reforms of the scale that are needed? Apparently not, for many Democrats.
But in any case, it is far, far, far too early even now to be quoting polls or demanding 'evidence' as though that actually means anything at this stage. There are months still to go, anything could happen, and every one of the thousands of polls that will be conducted will go out the window immediately after the Iowa primary results come. Biden's candidacy, for instance, is premised entirely on his apparently 'electability' (voters who have quite reasonably not been paying much attention recognise his name but nobody seems to actually like him, as his wife recently conceded), and his support will crater if results don't bear this out in Iowa and New Hampshire. It is worth keeping in mind that Clinton had a huge lead over Obama among African-American voters until he won Iowa, whereupon they all immediately began backing him, the candidate they actually preferred, the minute him winning over white people began to seem plausible. Strangely, the polls seem to suggest (again, grain of salt) that Biden supporters' second choice is by some distance Sanders (which might reveal a lot about how people who don't pay close attention to politics see things very differently than those who do). If that is true, and Biden flames out early, then... who knows?
If nothing else, I really do hope though that the Dems can at least do better than Biden. He is surely the single person in the Democratic Party who most closely resembles Trump. It is as though the cosmos is conducting an experiment to determine: are twitter Democrats self-aware? If Trump had promised not to take funds from fossil fuel companies while speaking at a fossil fuel company fundraiser, it would be just as plausible, but unlike with Biden, the Trump thread on here would have (rightfully) blown up. And the Party also needs (for once) to think beyond just the next election and beyond just the Presidency. Biden might even win against Trump (an exceptionally unpopular candidate), but to what end? Is he really what the Party needs over the next decade? And is he really the best person to ward off what will follow from the Republicans, a mixture of Trumpian rhetoric/contempt for the rule of law, but with basic administrative competence?
In 2016 there was a marked difference between Clinton/Sanders when considering a combination of both age and race. Older non white voters were overwhelmingly for Clinton and when combined with older white voters smaller preference for Clinton it made a huge difference as older voters are historically much more reliable primary voters.
