Actually, I think he's eventually going to be seen as a glimpse into the direction of the party, which would ultimately make him a run-of-the-mill dem in 15 years, not a radical. The question is if he could move away from the out-at-leftist-sea-all-by-himself position he finds himself in today by November. Ted Cruz and co. don't seem to garner the same kind of "radical" claims Sanders does within their party, and I think it's because the party rapidly moved with Ted Cruz into that territory. Sanders' challenge would be in getting the party to shift with him because, as it stands, a good number of dems could probably establish themselves as belonging to a totally different party than Bernie does.
For what it's worth, I'm not in denial about the difficulty involved in a Sanders GE. It would be tough. However, I'm personally of the opinion that either candidate would ultimately best what the GOP puts up, especially if that happens to be Donald Trump which, by almost every indicator, is what will happen.