If Sanders enters the 2020 race and mounts an even greater challenge to the Democratic Party I could easily see a split within the party with the centrists siding with the hard right from a real socialist challenge as just one scenario.

It does't make any difference: If Sanders runs again, win or not, I think the Republicans win the Presidency again. Maybe not Trump, and certainly not Pence, but some other GOP knuckle-dragger. This is exactly why Democrats in his home state are begging him NOT to run.