Was discussing something similar today, about the double standards a lot of people employ to filmmakers.
While some people are resolute in their conviction that there should only be one, unique, film made, it's inevitable that others - often the majority - will want to see the world that has been created expanded upon and another film created.
The issue then is that there is pressure for a filmmaker to make another film which, if good, leaves people clamouring for a further instalment. Eventually, we reach the position where a filmmaker turns out a film where the same people that were clamouring for sequel-isation believe that the concept has started to turn stale and elements of the film weren't as good as previous films.
It seems to happen with a lot of original concepts: people see the potential for further expansion, and want more from the concepts/world/characters have been created, until it reaches the point where everyone becomes sick of it and the filmmaker seems to be churning them out simply because they know it'll make a profit. For that reason, I believe that creating another film around a similar/the same concept should either be done as part of an ongoing trilogy or should be avoided altogether. It can work, of course, with sequels sometimes improving on the originals, but for the most part it leads to the film becoming a franchise which lasts longer than it reasonably should and eventually starts to taint the original and the entire series of films, with people saying they should have left it at the original.
I can at least understand it though. The double standard I really don't get is people who claim that releasing a trilogy (i.e. LotR) is a money-spinning venture, rather than just releasing a single film. These same people will then complain when a single film adaption of a lengthy text that they loved didn't cover every element of the story.