The trouble with fictional world-building is that keeping a consistent background requires constant nullification of all progress. A world that actually solves a serious problem once and for all would be good to inhabit, and even good to write about, but not good to sequelize.
- unless it takes on an entirely different serious problem, but then it would not be the same story, or the same world. If your plot moves in a circle, then you’ve created a world, and you can sequelize forever, but in vain. If your plot never turns back, then you’ve accomplished something, and you’ve created a history... but really you’ve destroyed a world, and you can tell that tale only once.