The movement stirred up by Sanders just wasn't strong enought to ge him the nomination. And if he'd gone independant and stood anyway, it wouldn't have been enough to win the vote, but would have been enough to put Trump in.

A Clinton presidency dependant upon a good contingent of Sanders inspired members and civilians is the best to hope for, which is why Sanders has called on his supporters to go for it. As also Noam Chomsky.
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The main danger now is that there's going to be an assumption that Trump hasn't got a chance, with the consequence that a mass of voters who don't much like Clinton will stay home in November, and that there's a surge of people who don't fancy Trump turning out to vote for him anyway, to give the establishment a fright, on the assumption there's no risk in that since there's no danger he might actually win. As was the case with a lot of the Brexit vote.