I voted for Harris. But in the first-past-the-post system, all that matters for a party is victory. That is how you judge the success of party officials. There is a whole class of extremely well-paid Democratic party officials whose ENTIRE JOB is to figure out how to select a candidate and sell them to the American people. That is literally their entire job.
The average voters is low-information, doesn't pay attention, and assumes both sides frequently lie. (And they're not wrong about the lying.) You can truthfully call the other side fascist, but the other side will simply say you're the fascist, and low-information voters can't tell the difference.
Voters have always been low-information. This isn't anything new. The entire reason we have primaries is that it forces candidates to actually try their hand at mass appeal and to take the temperature of the electorate. Democratic leaders kept Biden in far too long, til it was too late to hold a proper primary.
You can blame it on those who don't vote, but the truth is that most people pay attention to politics only tangentially. If you, as a political operator, didn't find a way to reach these voters, you have failed.
Does the blame ultimately fall on those who didn't vote for Biden? Sure. But the same is true of those who didn't vote for Walter Mondale. It doesn't mean Harris isn't just as big a failure as Mondale.