Well, the argument here is slightly more complex than that - that programmes which are dragged out for no good reason are rubbish - but it's a damn good article that you should read.

http://theamericanreader.com/the-cosmology-of-serialized-television/

One which really fits the bill but he doesn't mention is Desperate Housewives, which started as pretty much a murder mystery and descended into a soap opera being dragged out forever.