This is case is one I personally disagree with, but I do think the best argument against a second term was recently laid out in the Atlantic by Mark Leibovitch. Essentially that Biden is still with it now but might not be with it for a full second term or a much more physically grueling in person campaign now that the pandemic is subsiding.
My biggest argument against running a new candidate is the fratricidal nature of our recent primaries, particularly the 2016 contest, and without a unifying figure like Biden to rally around, we will see moderates and progressives duke it out for total control over the party. A Harris coronation seems like a similar mistake, since she has not risen to the high expectations we all had for her when she was picked for VP.
My personal favorites Pete Buttigieg and Amy Klobuchar are probably too moderate for the base and do not command the same kind of loyalty form black voters that Biden once did (and no longer does). Perhaps Booker can hold the black, moderate, and progressive wings together. Otherwise I see a dearth of nationally viable leaders able to pick up the torch, meanwhile the GOP has a cast of characters capable of moving past Trump and generationally contrasting with Biden if given the chance. DeSantis, Cotton, Haley, Scott, and Sasse are all interesting and potentially viable against a post-midterm Biden. None of them would be good news for the country.