Whenever I plan a multi-chapter fanfiction I make sure to have it finished before I start posting. I do this for several reasons. One: I want the story to be cohesive. No matter how much outlining I do by “chapter” four my characters may be at a place that necessitates a change in chapter one. And two: writer’s block can happen so badly that you have no desire to finish something you’ve started. But you owe the reader something because, to me, when you say you’ve written a part of a series, that is a contract saying you will finish the series.
This is why I have a problem with so many writers today. There are so many times where the final book is rushed or the author lost heart. You can tell because the plot is inorganic, the characters are not themselves, and the story is just plain awful.
And I know personally that it can take a long time to write a novel and write it well but for the love of all things holy do not take forever to get the next installment written and published! I should not be able to get a four year degree and work for over a year afterwards between the release of two of your books. (I am looking at, and glaring at, you GRRM).
In an ideal world my suggestion would be for an author to have a series complete before they release the first book. Or at least have outlines for each of the books and expected completion dates for them all when the first is accepted. I know this is not practical but boy would I appreciate it.
I know these things take time and that authors have lives outside of their writing but I also feel as though it’s rude to make readers wait so long for a book series. Imagine how many loyal readers will never get a chance to finish your series if they or you suffer from loss of sight, mental faculties, or death (much more likely if you do take over a decade or more to finish a series). It’s only fair that you try to keep that from happening by being timely.