The question of when to end a TV series is a perfect example of the tension between art and commerce. Many series are cancelled earlier than we’d like, provoking an outcry. Many are kept going past their prime, provoking disdain. But sometimes a writer gets the luxury of choosing the moment, and that’s what Jesse Armstrong, the creator of Succession, has decided: the much-anticipated fourth season of his HBO comedy drama will be its last. Of course, there’s no one ideal duration for a TV show. In the UK “Fawlty Towers only did 12 episodes” is often cited as proof...