I think a lot of people are misreading the Bernie vote. I don't believe his voters are much further to the left. I believe he has a vocal support group that is and stuff, but most people who are going in the voting booths and voting for him are not that in tune to politics to care.
I think a lot of his vote is "Not-Hillary," really. She's been around a long time. And yeah, there's also an obvious portion that is "Not-Obama" in there too (basically, Hillary promises to further Obama and these people are racist). It's not a huge group, though in some states they are (looking at you, West Virginia).
If Hillary was not ever married to Bill and she was a Senator and then SOS and decided to run for the first time now with the exact same message and campaign, Bernie would have gone home already. She'd be up by a fuck ton. Or a Younger Jerry Brown. Or an older Kamala Harris. Etc.
Young people just don't want any more Clinton or Bushes. That's all there is to this. The coming generation isn't moving far left. It is moving left, as it always does, but mostly on social policy (which hillary already represents even moreso than Bernie).
Young people are idealistic. I don't subscribe to the notion that people become more conservative as they age (and this has no bearing in data) but I do believe people become less idealistic and more realistic. Their positions become more refined. What was once "ban all fossil fuels" becomes "give incentives for alternative energy sources, reducing pollution, and moving away from fossil fuels."
So yeah, some young people right now might agree with Bernie, but I think in 10 years, their positions become more nuanced than his currently are. It'll be a shift left but not nearly as drastic as Bernie believes.
Hillary and Obama are where the party are today. But even though the party of 15 years from now will be further left than that, it won't be where Bernie is. It will lie with someone else, someone still a ways away from Bernie.
I know Bernie fans don't want to believe it, but I truly think a lot, if not most, of his actual voters are voting against Hillary and not for him.