I think when a show has story-arcs, it becomes harder to write something beyond 4 seasons. It's often why every new season will have a new story-arc. It's when a show that doesn't really have story-arcs that end up running longer, where the stories are a lot looser and not necessarily connected. The way I take it is, writers will get burned out quicker while writing story-arcs, and the show will run out of situations more quickly. Shows like the Simpsons are a good example of why a show like that can continue; no story arcs, and tons of pop-culture that can be used as the show continues to grow older. Sure, the writing may not be as sharp as it once was, but the formula is tireless, like a perputual machine.