Isn't Sanders the socialist who won 23 states and 43% in the Dem primaries with a comic store owner as a campaign manager? That should have been the first clue that Dems were in a bad place. Don't get me wrong, I liked him but he and Warren were absolutely unelectable and yet Clinton barely beat him. Even Dems were not inspired by her. I know I wasn't. At least he got Killer Mike to give him what was perhaps the greatest speech intro this cycle.
I don't think someone as far left as Sanders is electable in America as most moderates would see him as too far from the center and not vote for him.
The popular narrative in a lot of places is that Bernie could have beaten Trump. I don't necessarily agree with them, but it seems like a non-trivial amount of people think that it could have happened. Maybe if Trump's presidency causes a backlash we'll see Bernie on the 2020 ballot, but I'm not holding my breath.