With an embarrassment like Trump in office, assuming Hillary were to win the nomination, and lose the general, why on earth would Sander's movement fail to survive? More likely it would not only continue, but thrive through Elizabeth Warren as the probable Dem nominee in 2020, while the DNC comes to the tragically late realization that it made a mistake in its partisan backing of the wrong, less electable candidate, was punished for it, and is warned off of such meddling in the future.
As for Barney Frank, what does his opinion, like other establishment/pro-Clinton shills and talking heads, mean to me? Socialism is no longer the boogie man it used to be, there is nothing impractical about Bernie, or his vision (from a policy standpoint he's in the exact same legislative quagmire Hillary would be short of a sweeping revolution indeed with respect to the House and Senate), and there is absolutely no reason to settle for Clinton when a real progressive unencumbered by special interests is in the ring; likewise if she refuses to entice Bernie and his voters with overtures of compromise and representation in the event he does not win. You Hillarites keep saying 'not now'; and to that I say why the **** not? The biggest obstacle between Bernie and the White House is and has always been not the ridiculous circus of self-destruction that is the GOP, but a neoliberal corporatist claiming, and thus far quite successfully, to be a progressive.