I'm not on that train yet, but this thread is making me consider buying a ticket.
Bernie Sanders was not going to beat Donald Trump. You know how I know that? Because Bernie Sanders could not win the Demoratic party primary in 2016, and American independents and Republicans in the general election were not looking for a Socialist to lead them.
The only Bernie Sanders "what-if" that's worth pondering is this: "What if Bernie Sanders reaction to losing any path to victory in the Democratic party primary had been to put his full support behind Hillary Clinton, instead of string his campaign along for a few months while encouraging his supports to morph into Bernie-or-Busters?"
If the Democratic party moves to the hard left they're going to have a hard time winning in 2020. American elections are about personalities first, and policies second, so it's not impossible to imagine the right candidate pushing the party left. But to the extent that people (especially in the Rust Belt) are paying attention to policies they're going to see "socialism" as a huge negative.