Thanks for the question, Wyatt! I’m not saying he should necessarily resign right now. I think he’s been a good president. Based on his recent press conference, he is probably mentally able to make good decisions in the job. But I am pretty confident that him choosing not to run for reelection would significantly increase Democrats’ chances of beating Donald Trump.   After digesting the analysis from polling experts like Nate Silver, Harry Enten, Dave Wasserman and the guys at Split Ticket as well as so many center-left opinion writers who I generally trust (e.g. Jonathan Chait, Ezra Klein, Matt Yglesias, Eric Levitz), I’m pretty certain about a few reasons that this is the case:

1. The vast majority of voters think Biden is too old to be President.
2. He is consistently underperforming Democratic Senate and House candidates in the polls. 
3. Other potential presidential candidates (even Harris) are doing better in the polls than him once you account for name recognition.
4. Biden is currently trailing Trump, and he probably needs to win the national popular vote by at least two points in order to win. 
5.  Biden is less able to campaign hard and speak effectively in unscripted settings than all other potential nominees; these abilities are quite important to make up the polling gap.

I’m not going to rehash all the reasons I think giving Trump a second term is bad for the world. But just to name a few: 

1.  His economic policies (especially the tariffs, deficit-financed tax cuts which are too geared towards the wealthy, and immigration policies) would probably hurt GDP growth in the U.S. and across the world. 
2. I think him being in office increases the chance of the U.S. becoming an authoritarian state. 
3. I have more faith in a Democratic administration to prevent pandemics, effectively regulate AI and avoid nuclear war. 
4. There’s an outside chance his administration tries ban lab-grown meat, like Republicans did in Florida.