Duckman said:
I actually think that's why British shows are some of the best. Think about it, this show ran for how long, and look at the impact it still has.
But when you look at shows like The Simpsons, for example, it becomes so very clear that these US television shows get milked to the point where they just go on random escapades to save the world. Black Books I found to be too short as well, but would it have made it a better show if there were another 3 seasons?
I think the dichotomy you're trying to sketch is false. The premise of some shows doesn't allow them to run for long, that's true. Darkplace (which ran for 6 episodes, it had a brief run in Peru) certainly wouldn't have held up for more than 5 seasons (there's only so many times you can quote Shakespeare and cite it as "Hamlet, p. 54" or show 2 people facing each other in one shot and facing the door in a next before it gets old).
But that's not like an absolutism. Sure, Simpsons should've stopped years ago, and South Park should've stopped at season 9. There's no harm in them continuing either, it's not like the old episodes get less good because of the new ones. And they still produce occasional giggles.
So hell, it's not an absolute truth, nor an UK-US divide either (not like some UK shows ran for too long, hmmm? Are You Being Served? Dad's Army? 'Allo 'Allo!?).