> If the GOP was faced with a Manchin or Sinema they would be primaried and stripped of key committee positions (most notably Manchin's on Energy) to bring them into line.

The Democrats have 50 seats in the Senate. The Republicans have 50 seats. The Democrats have control only because Kamala Harris provides a tie-breaking vote. Sinema and Manchin are actively being courted by the Republicans ceaselessly. So if the Democrats try to whip them into line they can just withdraw their support from the Democratic caucus and then Mitch McConnell has the power. This isn't to say that the everything the Democrats have done to try to control the situation has been perfect, but these are the parameters they're working with. The story that the Democrats are feckless and happy to achieve nothing is the propaganda. It is spread both by idealists working with a fantasy notion of US politics and agents provocateurs who prefer the left stay cynical and demoralized.

Another element of this fantasy version of politics: the "Obama had a super majority" story, which echoes endlessly. Obama had this super majority for a number of weeks. First Republicans prevented the seating of Al Franken for months. Finally he was seated. Several weeks later Ted Kennedy has to withdraw entirely from his job because he was dying of a brain tumor. Then he died. During this time the legislative priority was passing the ACA. They wasted vast amounts of time with the Republicans playing a Lucy-and-the-football delaying tactic. But ever since defeatists trot this out as though Obama could have passed virtually everything during this brief window and the fact that he didn't proves his fecklessness. This is the propaganda. The Democrats have failed to achieve more not because they as a party don't want to achieve anything but because they are a relatively disunited party with different people pulling in different directions and they have only had very brief moments when they could exercise significant power. This is why everyone but Sinema and Manchin want to reform or abolish the filibuster: they realize that it prevents their ever achieving anything because at this point the Republicans will never work with them except on trivial matters.

Use Occam's Razor. It isn't secret cabals and epicycles that govern the political universe. It is exactly what you see: disunited people, some of whom are corrupt (Sinema and Manchin, e.g.) or stupid (Sinema and Manchin, e.g.), working with brief windows of opportunity within the campaign cycle. If you actually want leftist ideas enacted, working with a fantasy version of politics is worst way to go about it. It may make you feel individually virtuous and important, which is a great temptation for everyone, but it doesn't get anything done.

P.S. Obama could not abolish the filibuster. This is something the Senate has to do by amending the rules they are organized by. They can do it with a simple majority, but it has to be a majority of senators. And Obama's chief priorities when he came in were the ACA and enacting carbon pricing, not codifying Roe v. Wade into law. He managed to achieve one of these things largely because Nancy Pelosi gave up the House's more progressive plans when Ted Kennedy was gone and she realized further negotiation was fruitless. And despite herculean efforts on the part of all the Democrats in power at the time (well, except certain assholes), the circular firing squad devotees paint them all as feckless.