Politico makes the same point you raise. Many of Bernie's backers are not really supporting him based on his proposals or his ideology, but because they are anti-establishment. If Bernie falls out of the race, then they have no where to go but to Trump. Bernie's support is based on his personality and his combativeness, rather than his actual proposals as noted in this article.

http://www.politico.com/magazine/story/2016/05/why-bernies-bros-might-go-for-trump-213915

For all their divergent beliefs, Donald Trump and Bernie Sanders have each tapped into raw anger and resentment that is in some ways more emotive than ideological. The dangers for Hillary Clinton are clear. By most reasonable standards, she is as unimpeachably liberal as Humphrey was in 1968, yet she is equally distrusted by the anti-establishment left. She will need to guard against defections to an anti-establishment conservative who has proved every bit as deft as Wallace

And, like Carter in 1980, Clinton will enter the fall campaign with sky-high disapproval ratings, in no small part because her primary opponent spent a year casting her as an enemy of the common man. True, Trump is also wildly unpopular. But people tend to forget that Reagan was hardly more trusted when he unseated Carter than Trump is today—and that year, voters chose the candidate who represented a break with the status quo.

Citing survey data from earlier this year, political scientists Christopher Achen and Larry Bartels recently observed that “supporters of Mr. Sanders were more pessimistic than Mrs. Clinton’s supporters … and more likely to say that economic inequality had increased. However, they were less likely than Mrs. Clinton’s supporters to favor concrete policies that Mr. Sanders has offered as remedies for these ills, including a higher minimum wage, increasing government spending on health care and an expansion of government services financed by higher taxes.” Achen and Bartels attribute Sanders’ appeal to identity politics, particularly given his disproportionate traction with “disaffected white men.” In this respect, the Sanders electorate is not dissimilar from a large portion of McCarthy’s and Kennedy’s supporters.

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The historical lesson for Hillary Clinton is clear: Watch the left flank, because it could very well swing to the right.