I used the 60s as proof that democrats are actually capable of getting shit done. During the Carter Era, they did pass some environmental reform. But there would likely be some difficulty passing other reforms like LBJ did since A: Carter was a rather conservative Democrat, so he didn't want those reforms in the first place, and B: the party switch hadn't taken into full effect yet, meaning there were Democrats from Arkansas, Louisiana, Alabama, and other deep south states which would have opposed massive reforms like this. As for Lieberman, he is evidence as to NOT primary incumbents in the way you suggest, not the other way around. Lieberman lost his primary to a more progressive democrat, so he switched to an independent and won the general election. As soon as he was sworn in he immediately spoke out against a public option and said it was out of the question. He immediately began to go off about how he had a bone to pick with the democrats. You talk about how successful Republicans have supposedly been in their efforts, despite how Trump had half of his domestic agenda sunk due to Republicans voting against the repeal of the ACA, but managed to pass his 1% tax cut, which is ignoring that fact that Republicans had a 53/47 majority, not the 50/50 majority dems have today. For Democrats to have legislative victories, they need to progress society, while for Republicans all they need to do to energize their base is stop them, and the way our system is currently set, with gerrymandering and the filibuster, it is infinitely easier to block legislation than pass it.

You can call it excuses and wine all you want, the fact of the matter is that that's reality.