Derivative work is a little different than building a sequel though. A sequel may stand alone on it's own, but it also can take away from the original intention of it's prequel because it usually further builds out the universe or plot that took place in the original.
Movie producers often reach for sequels because they are guaranteed cash grabs and prop them up with nothing more than a shallow plot and a few A-list celebrities. To many, this takes away from the intent and meaning of the original.
I like a well-done sequel. As much fun as it is to step into a movie world for the first time and find out what's going on, there's a certain sameyness to the requisite beats to establish origins, establish personalities, establish locations, just a whole lot o' establishing going on. Again, that's great and all, but it limits the amount of time for the story itself, and limits the range of stories that can be told.
Look at all the superhero origin stories that have come out lately, and summarize to yourself the story in the movie, without including any of the establishing. Even in the best ones... in fact, to some extent, especially in the best ones... the story itself is quite simple.
The sequel will have a chance to spread its wings much more. The fact that so many fail to do so is... well... an interesting discussion on its own. But the really great sequels are often movies that had to be sequels, because they told a story too big to also have 45-75 minutes of establishing in there with them.
Derivative work is a little different than building a sequel though. A sequel may stand alone on it's own, but it also can take away from the original intention of it's prequel because it usually further builds out the universe or plot that took place in the original.
Movie producers often reach for sequels because they are guaranteed cash grabs and prop them up with nothing more than a shallow plot and a few A-list celebrities. To many, this takes away from the intent and meaning of the original.