Someone pointed out how much Hillary Clinton’s supporters disliked Barack Obama, until he was the official nominee in 2008. The (not so) implicit hope and expectation is that, Sanders supporters, like erstwhile Clinton supporters back then, would turn around. This rests on a key premise that may not pan out.
In 2008, Clinton was the nominee of the Democratic establishment. When Obama won the nomination, he became establishment. So the establishment voters did not have much inclination not to. In 2016, this logic applies to the Republicans, where Donald Trump, the erstwhile outsider, is now the establishment. Mitt Romney and other “elder statesmen” in the Republican Party are just bitter old men left outside the party establishment. On the Democratic side, things are different: Sanders was the outsider challenging the insiders, and now that he lost, he and his supporters are, to a large degree, still left on the outside looking in, without much invested in the future of the Democratic Party. This is especially true with regards to the independents who are not especially liberal who made up an important if overlooked minority among the Sanders coalition, who have NO investment in the Democratic Party.
Democrats may be able to absorb the Sanders voters IF they want to change their future. That means having to make concessions, real concessions that people can actually see so that they may become invested in the Democratic Party. This is not necessarily concession to “liberalism” or “moderation”: such unidimensional policy labels make little sense to the voters to whom concessions need to be made anyways, but those along actual substantive dimensions that make sense to them. But the truth is that the Democratic insiders really don’t want to change and they may be able to get away with it in the short to medium term, as there is no pressure on them to do so–at least they don’t think so since they see Trump as a joke. (In this sense, losing to Trump in November might actually be the best thing that could happen to the Democrats if they want to survive in the long run–NB: I am not invested in Democrats changing, but I don’t think they can survive unless they can change.)