Or he will look back and realise that American voters - and most people in general, really - don't really vote on policies but more on likeability, perceived "capability" and many other things.
This armchair analysis that "he hasn't done the work to reach out to those voters he needs" sounds good but is backed by virtually nothing. Those black voters didn't vote for him - so it must be that he had not reached out to them. No: he just didn't manage to sway them, especially the older ones.
Older black voters, for example, are a key Biden demographic, and virtually every study says that they are among the most conservative people in the country (in the traditional sense of the word, not that they are Republicans at heart). Of course they will go with the conservative candidate (again, not conservative in the American sense). It's very hard to overcome that with even the best grassroots organization. Especially when the mainstream news outlets constantly bombard voters with the idea that Biden is a safe, moderate, "more electable" candidate.
I'm sure Sanders made mistakes but overall, it looks like that contrary to the hopes of many, including me, he hit a glass ceiling in terms of support. It's easy to say he should build alliances like Bill Clinton and Obama: these were centrists who were perfectly acceptable to Wall Street, to corporate Democrats, to "moderates".