I believe it to be a fair assessment, and not because of my feelings or desire for change, but simply because I (and apparently some others further up this thread) saw basically the same thing happen.
I've worked closely enough to the Dem party/staffers/organizers that I've seen the same pandering play out locally, statewide and nationally in much the same way. It's mainly about playing ball to keep your funding streams up and to fill your little black book for the next cycles war chest. Certainly, some politicians are more genuine than others in the party but for the majority politics comes to be little more than moves to jostle purse strings from PACs, donors and the party itself. At the end of the day there's endless things that Dems could be doing to fight back against the takeovers of state level governments, school boards, appointee positions and the like, but they aren't.
We can say "but we're the one's who fight the good fight" but there's no keeping the genie in the bottle on the GOP side and Biden should have pushed his powers to the limit to gain every protection or have the courts limit his and future presidents power as a guardrail.
There certainly is always a narrative around these events, but Sanders was consistently polling as the most well liked politician in America for years around that time. There's no conspiracy around the ways that coverage began slanting as standings solidified, and every other contender in the race threw their support behind Biden (including Warren, which left everyone scratching their head.) Here's another article about MSNBC. If your actually interested that one has all of the measured instances laid out in google docs you can look at. I don't know any people alleging that the votes were manipulated in terms of ballot stuffing or anything, so if you mean that by conspiracy then I'd have to agree with you there.
Your assessment of political leanings in America also seems to be very skewed and there's a graph there that shows Dem/left sentiment at its highest point since 2008 during those primaries. Although it does seem to be ticking back conservative, but when Joe Biden still chooses to punch left on dem socialists after a failed insurrection that's not much of a surprise.