Fred Kaplan runs down the history of U.S. Presidents who decided against running for another term.
The situation with Biden is quite unprecedented. Just about everything in politics these days is quite unprecedented. A friend just asked me if America was ready to elect a female president. It was not so long ago that folks were asking if America was ready to elect a black guy. Everything is unprecedented until it actually happens. (And let's remember that Hillary won the popular vote. It's not like no one voted for her.)