He is bound to win Iowa and New Hampshire but I'm not sure about much else (I believe 538 suggested as much, too). South Carolina and the southern states in general would probably be a much tougher fight.
I gotta be honest. I love Bernie. I think he's fantastic and he probably reflects my political beliefs more than any political candidate since McGovern. But we know what happened there. I honestly don't think nominating a candidate that the party establishment absolutely hates will solve anything. I don't think you can sufficiently govern without being able to schmooze amongst the party elite and the politicians. Granted that Bernie has a ton of experience in Washington so far, but becoming the nominee means he is basically the leader of the Democratic Party. And I dunno that you can lead successfully as an outsider - our most successful presidents, by far, were supported by the establishment and knew how to play the political game exceptionally well. All our outsider presidents (Carter, for example, ran as the outsider that would go into Washington and clean up shop) have been relative failures. Hell, even Obama didn't have the rapport with Congress as some of our best, progressive presidents have had and we know how that has ended up.
Some people often compare Bernie to FDR. But it ain't comparable as far as I see it. FDR was a great, great politician and did some fantastic things for this country and is absolutely my favorite President but let's be honest: the dude was ruthless and did what he had to to win the election. His allies decried poor Herbert Hoover as a socialist and someone that was spending way too much. Of course, after elected FDR went on to spend even more and expanded the government because he knew full-well that Hoover was doing the right thing to counteract the Great Depression. But the narrative was that Hoover was an incompetent red socialist so FDR used that - doesn't sound much like Bernie, does it? And FDR was in control of the party machine, came from an elite background, was friendly with big companies etc. He flip-flopped and he pandered all the while have real, honest convictions that he kept close to his chest until he was in power. In that sense, he's probably closer to Hillary. I'm not saying she's the next FDR, but I think she's probably a lot more progressive than people give her credit for and she's doing what she's doing now (taking donations from employees of big banks and etc) because she knows that's what she has to do to win. I really doubt Bernie will be able to fund a national election on small donations alone. Not when he's up against a billion in Koch money.
I know the narrative of an outsider that can 'shake things up' is really alluring but I don't honestly think it works. I suspect Corbyn in the UK will face a similar dilemma. He's an old-school lefty that I think will find that having voted against his own party more than 500 times will probably hurt his chances of maintaining control of Labour. I view Bernie in the same category. Both are incredibly liberal (relative to their country) politicians that have been representing very liberal, homogeneous demographics that have stood for consistent policies while not being held accountable for those policies because ultimately they have never had greater national responsibility. If Bernie had been a Dem for thirty years instead of an Independent, I'd honestly say he'd have a much, much better chance.
Honestly? I'd love for Bernie to win the nomination. But even if he were then to go on to win the general (I only think he will be able to do that if Trump runs as third-party and horribly splits the Republican vote. Again, Bernie has popular policies but look what happened to McGovern) he would have a very, very difficult time getting his agenda through a party that would reluctantly support him and another party that will despise him in a way that we can barely even imagine.
What we need is to focus on local elections. On the House. On the Senate. Get progressives in there. Do that, and eventually we will be able to get hardcore lefties (again, relative to the United States...) in power.