In the article The Dangers of Franchise Longevity, IO9 contributor lightninglouie examines the trend for popular series, for example the Simpsons and the X-Men, to eventually lose the unique quality that made them special and popular and end up as stereotypical as the rest of the media morass in their genre.
“There is a big danger associated with longevity, and it’s not what you’d think. Often you hear about creators losing sight of what the franchise was supposed to be about and going off on a bunch of pointless tangents. But I don’t think that’s the real risk at all. The big concern is that the people who are in charge of the thing will turn it into what they thought it was supposed to be all along.”
LL singles out other series falling into this, but I suppose the one that stings the most, to me, is Star Trek, going from its origins as a pseudo-intelligent study of the human condition and optimism for the future, into a senseless space combat franchise full of 50-year-old sci-fi gag tropes. Read the article.