Newsom was on MSNBC singing Joe's praises, just like he would have done regardless, because Newsom wants to be president, but Newsom also polls worse than Biden. That's not hypothetical. Those polls already exist, and a drop in Biden's numbers isn't automatically a boost for Newsom. If Newsom thinks losing in 24 hurts his viability in 28, he wouldn't do it. And who could blame him? It's five months to the election.
The point is: It's possible that all of the options are bad. Biden was in the mid-forties before the debate and the thirties after. He went from near toss-up to probably losing if the election were yesterday/today. Newsom might out-poll Biden today, but that's not the contest.
The contest is with Trump. It's not good enough to poll better than Biden. You have to actually carry all of Biden's states and then some. If I'm Newsom and deciding whether to try to cobble together a five-month campaign and limp to November to save the DNC from itself and protect Amtrak Joe's legacy when I'm starting 15 points in the hole or run my own campaign against the likes of a Haley or DeSantis also-ran once Trump is term-barred, dead, or both in four years, I'm not taking a risk at the convention unless someone makes me very, very confident that I could win.
And there's the rub. Newsom wants to be president, and he'd love to be president in six months, but he's not going to take over a campaign that's already lost. If the party thinks Trump wins no matter what--not an unreasonable conclusion--why on earth would they burn their best shot of a rebound in 28?
Uh... okay, bro. You know that Donald Trump is also running in this election, right? Biden could be running single digits, and it still wouldn't change the calculus: If a Biden alternative can't beat Trump, they're not going to put an albatross around the neck of their political career just to lose in November.