The other day my brilliant wife and I were discussing sitcoms. We were discussing how sad we were that there were only two seasons of Flight of the Conchords, and how that seemed to happen with a lot of good sitcoms, which seems to be the most difficult type of show to do intelligently.
She said, "If they don't leave you wanting more, they leave you wanting less."
I was blown away. It's so true regarding television shows. You lament that Seinfeld ended because (despite the finale) the eight and ninth seasons were the best. But you wish The Simpsons would end, even though it was the best sitcom on television for a long time.
Once a show jumps the shark (a phrase taken from a Happy Days episode where the Fonz does just that, on skis, marking the decline of the show), it rarely turns back. A show like Saturday Night Live is an obvious exception. It goes through many ups and downs due to its changing writers and cast.
But what about fiction writers? Writers seem to be able to defy my wife's law. A writer can put out a bad book, or several bad books, and as long as they are willing to take criticism and experiment, the downhill path can begin to regain elevation. There are definitely writers whose careers follow almost a straight line down, like Chuck Palahniuk. But JG Ballard broke his slump of the disaster novels following Drowned World by reinventing his writing. A writer's next work can always be his or her best.
"If they don't leave you wanting more, they leave you wanting less." Still something to think about.