I mean, not trying to bash Biden, just being fucking honest here.
What did people expect from the guy who responded to years of anti-police protests that were basically asking "please stop killing us" that were responded to by police by becoming a police riot by... *checks notes... massively increasing police budgets nationwide.
Because he is. Even if we don't like him. Even if we think his handling of Israel is fucking disgusting. Even if we think he's too god damned old.
Trying to run someone new without any primaries now would almost assuredly leave us with a worse candidate and a splintered voting bloc.
Why? Because without primaries, it's basically in the DNC's hands to choose a new candidate, which they are totally allowed to do as a private organization. I know this because they never legally argued that they didn't put their finger on the scale for Hillary Clinton in court, they argued that as a private organization, the DNC is allowed to make its own rules and break its own rules. (The law was on their side, so they pounded the law, since the facts would have said they totally did put a finger on the scale for her.)
How many Democratic voters do you think are going to line up for a party-chosen candidate? What if they did "go into back rooms like they used to and smoke cigars and pick the candidate that way." I think a lot of voters would be real unhappy with that.
Even if the party chose to let their delegates choose a candidate at the convention (less than two months away), it could easily turn into an absolute shitshow and also split the party vote.
Biden is who we've got and we can accept that or fuck up our chances for any semblance of democracy even more.