First, it's not very smart to base your actions on what you have chosen to "believe". Beliefs reveal your prejudices, the things you haven't chosen to think through thoroughly, your weakly supported assumptions. And second, as Will Rogers once said, "I'm not a member of an organized political party, I'm a Democrat"... yeah, it's totally "believable" that Manchin, Sinema, and the other "moderates" are acting without anyone's blessing, just as it's totally believable that they are acting on principle.

The filibuster has not only blocked progressive actions, it has blocked reactionary GOP attacks on existing progressive policy and law. If nearly half of citizens strongly oppose the creation of new government law and policy then riding roughshod over them will probably cause a lot of problems, will probably create backlash that can easily undo the progress. The key to making progress is convincing enough voters that it's the right thing to do.