Yes, Biden shouldn't have run, or should have dropped out earlier.
But
1/ The Party could have made a different choice, sooner.
2/ Harris wasn't the best choice, and maybe wasn't the best choice given the situation. She was the one Democrat who could be saddled with all the negatives folks had about Biden's administration, and who could be asked "who didn't you do all these things already?"
3/ She isn't charismatic. She isn't good on her feet. And she seemed to avoid interviews because of that, and didn't do so great in the ones she had.
4/ None of that really is the key issue. Democrats yielded the floor on populist issues that they should own. They did this because they allowed Republicans to make it all about identity politics, and create a binding of being a certain kind of person in various aspects of your lifestyle and outlook, with being in their MAGA movement. By making the movement its own thing, they were able to capture people who wouldn't otherwise be "Republican". They managed to paint Democrats as the party of rich fat cats and corporate power, and yet still paint them as "socialist radicals" at the same time. Until Democrats embrace that you can identify as the kind of person MAGA attracts, but that doesn't matter because on the issues you are a Democrat, they will keep losing. This was the problem with Hillary Clinton, who she would be a great president but was a terrible choice as candidate. It's the problem with how they treated Sanders. It will keep being the problem as long as Democrats think MAGA folks are deplorable instead of tired of being disenfranchised economically since 1972. Many of the non-social things MAGA appeals to are historically Democratic issues. And the social stuff is all culture war that has little connection to what POTUS does.
The Republicans were no better at understanding this than the Democrats. They thought Trump had no chance in 2016 because of it. Trump took over the party. If Democrats don't reform right now, they too need a takeover. It will be a populist left firebrand, with moderate-sounding culture wars views. It will be a 45-year-old Sanders-meets-Clinton-meets-Obama. It won't be such a problem to be more left as UBI becomes a necessity. The economy is going to boom, but more and more people will have no place in it. The AI future is a future of fervent capitalist creative destruction on top of a social contract for socialist sharing in the general commons, and out to all people, of the fruits of that economy. It's a low regulation, high redistribution economy, with meaning created in a new way that delaminates life-value from work. This is primed to be Democratic Party landscape, if they wake up. They need a generational reconceptualization, the way that Lee Atwater did and Trump is the culmination of. The Democrats need to become the party of the post-industrial-capitalist, post-corporatist, AI-based future new society. The need to be the party of pro-growth, pro energy usage, pro tech as a path out of debt, pro universal social equity participation in the era of abundance. The person who will lead that first way is in their 30s now. The person who will do it through and through is a teen.