When should shows die?

Many times in television, TV shows air way past their expiry date. That is to say, it got old, it got ridiculous, and it's still airing. Other times, genuinely good shows have been cancelled despite a good following. Sometimes, a show is cancelled after several seasons, perhaps due to lack of ideas and not wanting to 'milk the cow' so to speak. Other times, a show is done to death regardless. This article should explore why. Explore what makes both happen. WHat are the network's incentives to keep a show running? Do ideas have anything to do with it, is it what they believe audiences want? An agenda? All three? When, ideally, should be the time to cancel a show or keep it going.