Joe Biden is 78. He only ran for president because he (correctly) thought he was uniquely positioned to rid the country of Trump. He succeeded; to some extent, what actually happens during his administration will be an afterthought. He may well not run for a second term for that reason.
Biden doesn’t have to worry about his legacy the way most presidents do, because his will be assured the day he takes office, and because he probably won’t live long enough to watch the debate. That gives him a freedom in office that most presidents don’t have. If the left bashes him unmercifully on Twitter, as it almost certainly will, what’s it to him? He can just live in the moment and try to do what’s right for the country, not himself.
Here’s hoping he takes full advantage of the opportunity.