Bill Ackman just came out backing him

I'd say it's a buy at this point. Here's a couple of scenarios, not particularly likely.

Dean Phillips could surprise. Primary voters are quite nervous about Biden's age. Dean Phillips is a handsome man, actually has a personality, and is not 80 years old. If he ever gets known, a big if, I think he could cause Biden trouble. Not saying it's at all likely he wins, as he's not even going to be on the ballot in Nevada, and Biden will win South Carolina no matter what, but at 3% I'll buy a longshot. Probably, he needs Biden to have a McConnell moment the week before a key primary, but also have it not be bad enough so that Biden actually drops out of the race -- in which Dean Phillips won't be the nominee either. He won't have the money or coverage to compete seriously. I could see him rising in the polls in New Hamp the week of the primary and end up getting 15 or 20%, and that's probably the ceiling. But voters could be more fed up with Biden's age than people realize.

When Joe "Badakathcare" Biden's wheels fall off, they are looking to California, not Minnesota, for the heir apparent.

The only way he wins is if Biden drops out or is forced to drop out of the race for whatever reason. I think this scenario is probably around 50% of the reason why Phillips announced a primary challenge in the first place. He is gambling on being the only one set up with a team, ballot access and some fundraising in what would effectively be an open primary.

The other 50% are probably a mix of frustration with being an unimpactful political-nobody type backbencher in the house and somewhat idealistic/grand/delusional ambitions plus the fact that even if he loses he'd still be rich and set up to run for Governor or Senate in 2026.

So, what's the chance of Biden leaving the 2024 presidential race? Probably <5%.