Part of the problem I see is a major progressive generation gap at the top levels of elected government. Not even proper leftists, mind you, just progressives. Bernie Sanders is even older than Biden. The next oldest prominent name is AOC, who is almost 50 years younger and is only barely constitutionally eligible this year (by less than a month, even). There's probably someone political wonks can name in between those two, but they're not household names and would be unlikely to be able to spin up a campaign this quickly.

Decades of centrism have gutted the Democrats of anyone who could possibly stand up right now.

Maybe this is the difference between American and Canadian politics? In Canada, it isn't unusual for a Prime Minister to step down and another person gets selected as Prime Minister. Not voted, selected, by the party in power. Biden would have been rotated with someone else by now.

During Brexit, that happened 4 times in a row! David Cameron stepped down and Theresa May took his place. She stepped down and Boris Johnson took her place. He stepped down and Liz Truss took his place, she stepped down (after just 40 days settings the record for shortest time as prime minister) and Rishi Sunak took her place. The general public didn't vote for these people to be Prime Minister. They were selected by the party in power.

That's why I'm sort of confused why Democrats don't just pick someone else. In a parliamentary system it wouldn't be a big deal. The general public are voting for a party, not a person.