It can be argued as shared fault.
By, without vote/primary, unilaterally selecting a candidate to go on the ballot an unelected bureaucracy jammed up the election. Unfortunately in USA, it doesn't work how you propose, whether you appear on ballot is only up to democratic choice if there are primaries, if not an unelected bureaucracy selects the people that actually go on the ballot and due to dynamics of our voting system virtually ensure those will be the options.
In most states you basically have Democrat, Republican, maybe Libertarian party nominated candidate on the ballot and that is it. Writing in is throwing your vote.
I would argue we probably could fix this with write-in only and some sort of ranked voting kind of system or similar, but as it stands a large part of the election process is vulnerable to anti-democratic processes and this played out in Trump's favor last election.
Which I will completely accept as true. They didn’t.
From here, there are two branching paths. Did the Democrats put up someone who was actually worse than Trump? As in, are we better off than if the November election had gone the other way? Or did the Democrats have a better candidate who just wasn’t better enough to win? (Fully understanding that this is a very subjective question.)
It’s my firm opinion that it’s the second one. Harris would have been a better President. (So would Jeb! Bush, Mitt Romney, the festering corpse of Richard Nixon, or a frog snatched out of the Tidal Basin.) In which case, giving Democrats any blame for the outcome requires the people who voted for the actual winner to have no agency. They were presented with a choice and they selected the worse one. That’s entirely on them.