It's a difficult call - but Sanders could win California and at that point, I believe that many "super-delegates" would rethink their commitment. The USA is faced with 2 extreme right wing conservative candidates and a social democrat.
If the 2 conservatives run against each other, it will be an attack campaign that slurs and insults and fear mongers. Neither Trump nor Clinton is capable of leading the USA except into the inevitable obliteration of Wall Street. If either of them wins, the people are likely to elect a contrary house and senate. Both Hilary and the Donald are disliked and distrusted. If Sanders runs, he will likely win and will also have more Democrats in the house.
Sanders could probably influence the United States to become a true world leader rather than the pugnacious bully practicing military and financial terrorism that it has steadily become since Regan.
He beat her in a poll, so why should he not be in the race? How are these two things relevant at all? He has an agenda and knows how to speak to get people to listen, while genuinely coming across as a well-meaning man. In so far as we've seen, there's no reason for him to drop.
If Sanders beat Clinton, he should be allowed to continue campaigning. Why should someone drop out because they are hurting someone else's chances of winning. I would like to think that the better candidate would win regardless of how the polls might look at any one particular moment in time.
Sanders has no reason to still be in the presidential race. He has won very little, and he certainly is not going to win the Democratic nomination. He is just continuing to split the Democratic party by staying in the race, and they will need all of the support possible for a tough race against Trump.