I only care if Biden drops out in that I want to see Democrats' chances of winning the White House, Senate, and House maximized, because the Republican Party has openly broadcast its desire to establish a fascist dictatorship.
I have not seen a ton of evidence that Biden dropping would advance that cause; although polling coming out this week, as the media has negligently and relentlessly focused on Biden's debate performance above any and all other stories, has now maybe started to suggest that. Lots of polling has indicated that a majority of voters, including independents and even Democrats, want Biden to not run. That's great, but we all know when you start identifying alternatives as opposed to the idea of a generic replacement, support plummets. This is evidenced by other options polling essentially the same. So in that sense I am somewhat agnostic. If it becomes clear to me that Biden stepping aside meaningfully increases win probability then I'd be all for it, I'm just unconvinced that it does, even now.
Kamala Harris is in all likelihood the nominee if that happens. Base your takes about whether you think Biden should step down accordingly. She is logistically by far the smoothest option and was elected by the people of the United States of America to be the next (wo)man up if, God forbid, the need ever arose. There is no reasonable argument from within the Democratic Party other than "eh, I just don't like her;" which sets off a shit show of a contested convention that I have no doubt Democrats would find a way to bungle.
If Biden does step aside, Harris's candidacy is a political damned if you do, damned if you don't situation. Obviously there are undecideds/independents willing to give Democrats a second look with a younger candidate. However, we saw a black president break a good chunk of white America's brains and we see similarly inspired posts in this very thread discussing her candidacy. I have a hard time thinking a black woman retains the support in the suburbs and among white seniors that Joe Biden has, nor is she likely to be significantly additive when it comes to black and latino men. Conversely, black women are an integral part of the Democratic base and passing over a black woman who is very literally heir to the throne is not going to go over well. I don't know how much she nets over Biden, if anything.
Gavin Newsom exudes rich, California limousine liberal. While I think he is very good in interviews, and is clearly a sharp guy, and has a decent policy record, that will be the message and it will stick with receipts like the French laundry debacle. He will be a non starter in middle America.
Gretchen Whitmer and Josh Shapiro at least bring the potential to shore up key states, but see the point above about passing over Harris and the reaction from black women in the base. Whitmer also has to deal with the burden of being a woman. Shapiro probably has the shortest list of negatives, but as just a two year Governor of PA, we don't know what we don't know in terms of vetting. And I frankly struggle to see the Democratic Party passing on a sitting VP for a two-year Governor.
Anyone else is small time and not worth considering.