Here are two examples of an original film that was a success, garnered sequels but the same creative people involved in the first film remained for the sequels. First I will mention what is fact about them then second I will comment what I imagine may have been the case for other areas.
Back to the Future of course was a huge success and Universal was going to go ahead with a sequel with or without Zemeckis and Gale so they agreed. In this case they were very fortunate to even be given an ultimatum. It could have easily been a case of being told we're going to do a sequel without you. Goodbye.
Star Wars was such a huge success that sequels of course were made. But the interesting thing about this case is that George Lucas made a deal with 20th Century Fox that he were to retain all merchandising and sequel rights of the franchise and in turn he would decline his director's fee.
In the above two examples those involved were very fortunate that the original film they worked on was a success and enough of a success that they were able to remain involved in the sequels. Now for the parts of those two examples that I imagine might have been included in those facts, remember I'm not claiming that what I'm about to write is fact, just my musings on what other things may have happened.
For Back to the Future it may have been a case of Universal seeing that there was a potential for sequels and they really always (after Back to the Future was a success) wanted Zemeckis and Gale to be involved but in order to make sure they may have gone for the hard front in telling them a sequel was being made with or without them, hoping that they would say yes but if they said no they might be in a bind unless they found people who could make it work.
On the other side of this it may have been the case that Zemeckis and Gale evaluated the situation and may have decided to remain involved with the sequels because they may have thought there was a possibility the sequels may ruin the first film and so in order to steer the ship correctly they said yes to being involved.
Who knows whether this was the game both parties were playing but I assume that this kind of thing happens in the big leagues behind the scenes. A game of back and forth, showing your left but giving your right, holding cards close to your chest, a game of chess where you have to use the right strategy at the right time in order to come out on top.
For Star Wars 20th Century Fox may have looked at it's success and figured that George Lucas was a talent they needed to retain as his involvement would likely result in more success. But perhaps the fact that Star Wars had a hard time during production the studio may have had a little bit of resentment towards George and if he didn't do the sequels then perhaps they would still try to find someone who could.
However they may have thought that if George were to say no then they could be searching for years for someone to do the sequels so we should try to keep George on board and how do we do this? George comes to the table and may have said okay if I'm going to be involved in the sequels I want to make sure I have creative control so I'm willing to forego my director's fee as long as I can retain sequel and merchandising rights. He may have known at the back of his mind that 20th Century Fox would not think that the merchandising rights were a big deal, they may think that the sequel rights are a big deal but they may have decided that that was also a small price to pay for more sequels.
George may have seen beyond the film itself and recognized the massive commercial opportunity with selling merchandise based on Star Wars and so he wanted to retain control over that as well as sequels. He of course already had sequels in mind so for him it may have just been a case of now they see the potential of the universe I've created so they will allow me to do the sequels. George was also in a very powerful position because not only was Star Wars a huge success commercially but also for the film industry itself. In my opinion it seems that George gained even more creative control and 20th Century Fox became just a studio that presents Star Wars as opposed to one that controls Star Wars like they would control any other film under them.
I guess as a filmmaker questions that have to be asked of oneself are:
"If I am really passionate about this story, if it's a success, would I want to be involved in sequels and have creative control?"
"If I am worried that any sequels that were made without me would ruin the original then should I go ahead with this film or should I leave it for now until I have power to retain complete creative control?"
"If I really want to retain creative control over any sequels for this project should I look at doing films that I may not really have much interest in so I can gain financing for the projects I really want to do?"
I would probably go with the third question. It seems a lot of filmmakers go that route. Christopher Nolan with The Dark Knight Trilogy may have decided that in order to do The Prestige he had to do The Dark Knight, in order to do Inception he had to do The Dark Knight Rises and now that he's done The Dark Knight Rises he can do Interstellar.
If this is the case then I don't think he didn't have any passion or interest in the Batman films I just think he may have seen the best way to get personal projects made was to work on sequels to Batman Begins to ensure the characters established are treated the way in which he felt they should as well as gaining leverage to do his personal projects.
I can tell you this as a complete fact - I wish I was a fly on the wall in every meeting that takes place for films
Writing requires much time and efforts, like any other job. Do you value your time and efforts? Besides, you're not writing to put it on a shelf - you want others to read it, don't you? Writing as a hobby will not make many people read your work.
And sold it to Joel Silver.