5. The X-Files
Here's a general rule of thumb: when the popularity of your show relies almost exclusively upon the interplay between the two main characters, you kind of need those two actors to star in the show. When one of them decides to throw in the towel, just take the show behind the shed and put it out of its misery. Anyone can tell you that The X-Files isn't The X-Files with David Duchovny, but apparently the network executives, in their infinite wisdom, needed two seasons to reach that conclusion. The X-Files peaked creatively in its fifth season, Duchovny became only a recurring character after the seventh, and the show sluggishly dragged itself along like a wounded animal, finally cancelled after nine seasons.