Sanders' proposals are so far-left and so socialist that many Democrats will likely shy away from them. Even assuming Democrats could win the House AND Senate, odds are, he could not muster a majority for major tax increases and free college and student loan forgiveness and the rest of his "social justice" platform.
(And the young people, while vocal about supporting Sanders, are not turning out en masse to vote for him. Primary election turnout for young people is less than it was in previous years. One potential caucuser in Nevada complained that the "lines were too long" so they left before voting, entirely missing the point of Democracy. Apparently they were "feeling the Bern" at a rally, but when push came to shove, well, there was something more interesting to do.)
This of course, begs the question whether Hillary will have any better luck if she were elected. Republicans hate Hillary with a passion, or at least project that image to their constituents. A lot of this, like the Obama-hate, is just posturing to get the "base" all riled up and voting. Most Republicans (like most Democrats) on the Hill don't really give a shit about Benghazi or your latest social issue. They care about who is giving them money, and the people who give them money are only interested in money issues.