2016 would have been a cakewalk for the Democrats, had they conducted the primaries honestly. Sanders would have won the primaries and he could have won the POTUS elections by a landslide. But the establishment Dems had other ideas and they rigged the votes so that Hillary could win. And in the end, it turned out to be a blessing in disguise for Trump.
Assuming Sanders won the primaries, he would have lost the presidency by a landslide -- he is much too far to the left to be reasonably electable. This is also true for the majority of candidates running in 2020, their policies are way outside of the mainstream, and will only win the very solid left states. The unfair 2016 primaries caused many Sanders supporters to either stay home or support Trump out of spite in 2016. Had the primaries been fair and Clinton won the primary fairly, her chances in the general election would have been much better.
I can't agree with your argument. Sanders may be far-left, but all of the opinion polls had him leading over Trump by double digits. And most importantly, Sanders was much more popular than Hillary in the rust-belt and the other swing states. On the other hand, Hillary derived most of her support from the deep blue states such as California and NY.
And that is the reason why Hillary lost, although she got more votes. Her support came from the deep blue states, which were always solid Dem. She needed to win states such as Wisconsin, Pennsylvania and Ohio.
