Re: Bernie Sanders...
Not only is he the most popular politician in America:
...but he would have won against Trump last time, too:
That is... questionable.
First of all, keep in mind that while Sanders may look good in the polls (both now and in 2016), he has never been subject to any sort of major attacks from the republican side. Its easy to look good when nobody says mean things about you. I suspect that had he won the nomination and the republican dirty tricks started, his popularity would have started to drop.
Secondly, he had a problem with minority support. One of the reasons Clinton lost is because some of the minorities who voted for Obama decided to sit out of the election. And Sanders had even less support among black and latino voters than Clinton did. If Sanders was the candidate, you'd probably see even more potential democratic voters deciding not to bother heading to the polls.
Lastly, money. For better or worse, elections are expensive. Yes, it would be wonderful if the U.S. could fix its elections so you don't need hundreds of millions to run a campaign. But, until that happens, you have to fight the election with the rules that are in place. Sanders does a good job at earning money from small donors, but he wouldn't be getting the big donations like Clinton did. You could say that he's taking a stand on principles, but if the end result is that you can't buy air time to run campaign ads because your cash has run out, you're going to be hampering yourself in the election.
The argument the centrists give is that "opposition research" will hurt him, and the Republicans will paint him as a communist, but the data indicates that instead of making Sanders look bad, screaming about socialism while pointing at him just makes socialism more popular.
First of all, what "data" are you referring to, that attacking Sanders would make "socialism more popular"? Just the fact that "gee wiz, polls show support for single payer health care" doesn't mean that, once the republican spin is in place, people (especially moderates/independents who need to be won over) won't stop thinking "socialism is a dirty word".
Secondly, keep in mind that not all of the attacks against Sanders would have been due to his political positions. If he had won the primary, you would have seen all sorts of reminders from the Republicans that he was a rally where people were chanting "Yankee die" (and I can't really seeing that being something a lot of people would say "Yeah, I agree with that"), while Fox news would eagerly start to air news stories about how Sanders was a thief (because he stole electricity from a neighbor when he was younger.)
Really, I think some of the centrists would rather just lose than shift left, and other are caught in an ideological echo chamber where "left-wingers can't win, only centrists can win" is considered "just common sense".
Many people think you need to be 'centrist' (in the Democrat's case, center-left) because you need to win over independent voters in addition to maintaining your core voters.
If you were an independent voter, and your choice was against a far-right and a far-left candidate (neither of which had policies that you really liked), then why vote for either of them?