If you said that five years ago, I would have agreed with you. However, I was pleasantly surprised by how well he did in the 2016 primaries. I had him pegged as another Howard Dean, but he did far better than that. I think things have really shifted in this country; likely it's a combination of unsuccessful, unpopular wars, the recession earlier in the decade, the threat of climate change. Another thing that must have played a role is the ascendancy of a new generation where all those things have hit them smack dab in the face and they can't just ignore them because they were living as comfortable adults sure of their place in life.
Is it enough to win a general? I don't know, but I'm willing to try, and argue that the exceptional circumstances we're living in make that more of a necessity rather than less.
The way I see it, if we don't address certain issues, that paves the way for more candidates like Trump or worse in the future. I'm not sure I'd count it as a wing if we get a Biden presidency and 8 years later we get Trump 2.0 because he didn't do anything about serious problems (like say, lead in the water supply in Michigan).