Sanders vs. Trump would have seen Sanders lose by a larger margin. "Young democrats" is a tiny demographic that can't swing an election, openly embracing the "socialist" label would have "excited" a lot of people all right - to vote against him.
Please. Sanders would have played Trump in the debates like a fiddle. Trump can shout "he's a socialist!" over and over again all he want, but he wouldn't have any of his ammunition like he did against Clinton - the emails, the speeches, etc.
Sanders is old. Old people have skeletons in the closet. Sanders wasn't a big deal long enough for the press to have really dug in and started dragging those old skeletons out.
Sanders had a similar message as Trump, just a bit different flavor. That resonated with the voters. I think Sanders would have grabbed more electoral college votes but to win, no idea. Hindsight and all that