Reading Nassim Taleb's article [1], I feel politics will become more and more extreme in the coming years.

Where it will become the far-left vs the far-right. And honestly, the US "far-left" isn't really that extreme compared to some European parties.

1. https://medium.com/incerto/the-most-intolerant-wins-the-dict...

But isn't that because the electorate itself doesn’t support such candidates? The Dems going hard left will make it easier for Repubs to be electable.

Only to the degree that the Republicans don't go hard right.

I think their appeal is broader than conventional Democrats. Sanders won northern rural Democrats over Clinton in the primaries, which seems to indicate a precise rift that hasn't been fully elaborated: https://www.nytimes.com/elections/2016/national-results-map

Clinton was the worst possible broadly electable candidate dems could have put forward. Gephardt would have been better. She was and remains a devisive figure. Dems who broke didn't as much vote for Bernie or Trump but voted against Hillary.