Although I generally agree that Sanders would be a risky candidate, I don’t think it’s obvious that he couldn’t win or that the election would end up as a blowout for a couple of reasons:

1) The big reason is polarization and negative partisanship This is also why a Trump or a Cruz nomination would not lead to a blowout loss for the GOP against Clinton. In both parties, voters are much more ideologically cohesive and dislike the opposing party much more than in the past. On the Democratic side, there are far fewer conservative voters who would prefer a Republican to even a very liberal Democrat like Sanders.

On the Republican side, there are far fewer moderate to liberal voters who would prefer a Democrat to even a very conservative Republican like Cruz (not sure how to classify Trump here). Therefore defections would likely be far smaller than in past elections like 1964 or 1972 when one party nominated a relatively extreme candidate.

2) The second reason is that the GOP may well nominate an extreme or high-risk candidate of their own in Trump or Cruz. Even Rubio is no moderate. In fact, his views are more extreme in some ways than those of any recent Republican candidate.