I've checked out on the news and much of this section of the boards the last 18 months or so as it was detrimental to my mental health (seriously, I've been much happier focusing on other things). But this is high profile enough to hit my radar.
The Democrats had a chance here with Biden stepping down. Biden is very unpopular. You can't fight those numbers. Him giving up the nomination offered a chance to put in a much more charismatic and likable candidate. Instead they're poised to move on to Harris, one of the few politicians out there almost as unpopular as Biden.
Also, Harris is not a great candidate apart from her association with an unpopular administration. She's not a dynamic personality and she's had high profile struggles with organization and leadership.
There were several options out there with the potential of poaching moderate, largely affluent Republicans who don't care for Trump or at least swaying a battleground state. Had they picked one of them, they'd be in a much better position. Moving on to a wildly unpopular California liberal was about as dumb a move as they could have made. They might have been better sticking with Old Joe.