If Sanders actually wins the nomination, he is not going to win the general, period. It would be a disaster for Democrats in he won the nomination.
That depends in a large part who's the GOP nominee. Sanders may be better-equiped to take on Trump than a conventional politician like Hillary in a year where the "establishment" is not a very popular word.
Just a few months ago nobody believed Sanders can make a showing and look where's he know (same goes for freaking Trump).
FTR: Though I support Bernie, I still think Hillary is going to be the nominee at the end.