I'm not saying you're ignorant of this history, chunk, I'm saying you're ignoring it. I don't like the tendency of pols of both parties, particularly Democrats, sucking up to corporate donors, but it is a reality of politics that money wins elections. It is true that the Democrats lost their way in the 70s and 80s, overreacting to Reagan's victory. But, blaming it all on the Democrats, and centrists in particular, is ignoring the realities of the times - and ours as well.
I use Wikipedia as a source here because it is available to everyone, and is curated, so tends to be a pretty neutral source for information. If you read through that history, you understand my point. Democrats in the early 20th Century were very different than Democrats in the 30s, and even more different to Democrats in the 60s. The party has gone through many transformations, but it has remained, since the advent of Teddy Roosevelt's Bull Moose Party, the party of progress. And that goes for all branches of the party.
Joe Manchin and Jon Tester are Democrats, and they are elected Democrats in States that are overwhelmingly conservative. Did you see Tester's speech at the 2016 convention? Tester was on Rachel Maddow the other night for "the Interview". If you watch that, he knows how Democrats can win in rural America, and that message should be listened to. Tester doesn't agree with all of the Democratic platform, but if he insisted on purity, his would be a Republican seat. Same with Manchin. Same with a half-dozen other Democratic Senators. The Democrats would not have the majority in the House without a goodly number of Democrats who just won seats in conservative States. Without the majority, nothing gets done.
That's why many of us, myself included, chafe at your pursuit of purity within the party - it is self-defeating. It's not that we don't hold many of the same beliefs and desires for reform, but, speaking just for myself, I don't believe your approach will keep majorities or win the White House. As I have said before, there is a formula that I think will win, and it happens to be the same three issues that Tester talked about with Rachel Maddow: Healthcare, Education and Infrastructure. That formula works in Rural, Suburban and Urban communities. Push those and I think Democrats will gain seats in both Houses in 2020, especially in rural areas.
I also think that you are ignoring the reality of the makeup of those Houses. The majority of seats are controlled by rural States. It is true in both the House and Senate, although worse in the Senate. In order to reach voters in those jurisdictions, one has to address their needs. They are more conservative than most Democrats, but if they can't be persuaded, they will remain a roadblock to progress.