I think enough Americans will vote for a black person (Obama got a majority) and a woman (Clinton got a majority) to win the election.
But there are some Americans (a minority) who will not and who cannot be persuaded to vote for a black woman. ALL of those folks also don't vote Democrat and never will, so there's no point trying to appeal to them.
I think Harris will win a crushing majority against Trump, who is reviled by all sensible people. Fortunately, there are more sensible people than non-sensible people in the US. All we have to make sure of is that the voting majority is more evenly spread than last time.
The only way the Dems lose is if they don't believe in themselves, or if Trump (and his Russian gang) cheats, like last time. Harris will crush Trump in a debate. I look forward to seeing it happen.
I like Warren, Buttigieg and Biden as well, but I see Biden as the one who will find debating Trump hardest, because it will seem like two ancient folks fighting it out. I want the Dems to present a generational contrast on top of the policy debate.
Biden's experience as Veep would block the criticism that Buttigieg lacks it, for instance.