There's a danger that maintaining a franchise just for the sake of the green backs, erodes whatever artistic integrity it once had, becoming a former shell of itself, a name only in appearance. It could be said that as a familiar marketing title, the audience will come no matter how poorly written. The writer or committee of writers really don't have to make much of an effort in telling a good story.
A colleague of mine is so facinated with the Rocky series, that he doesn't care if Stallone does part #7 to infinity, the same could be said to those that liked the Halloween series, regardless of the over the top plotlines that movie laid out in its sequels. The audience today are in love with the concept not the execution. I think one commenter from another site said it best, "as long as I see Jedi's fighting and space ships exploding thats all that matters". That's an honest opinion I can respect, as opposed to the blind loyality (excuses) given to piss poor sequels.
There are two issues here, audiences low expectations and lazy writers.
I enjoy watching the characters and their adventures, but is it like too much to ask for people in charge to make a decent sequel, and the audience to demand more?