I suppose that in the end, what I am saying is that authors owe readers what they owe themselves, which is the truth. Yes, I know some readers are not happy with this answer. They feel the author owes the completion of a series in a way the reader wants. Or they feel that the author, once having begun a series, owes the reader continuing to write in that series a book a year until the author is dead—and perhaps beyond the grave if there are any notes or other hints about what the author might have written after death.
But is this really what you want? You want an author in chains to an idea formed nearly a lifetime ago, when the author was a completely different person? You want someone going through the motions, writing about a character they now hate or telling stories that don’t matter to them anymore? I don’t. I really don’t. There are characters I love who I would like to hear more about, but if the author isn’t interested in writing them, I can go find fan fic or write my own. And that’s often better than what the writer would have written if it’s not whole-heartedly engaged.
I’ve felt series and characters were ruined when authors kept writing after the spark had gone, so that’s the worst case scenario for me. I’d much rather an author reinvent their own writing and try something new than go on beating a dead horse. Or take a break and see if they have something to write about later that is going to fulfill them more.
Remember that one of the chief pleasures of reading is the anticipation. In some sense, reading to the end of every book is going to be a disappointment. It’s why as readers we all have the tendency to slow down as we come to the end. Some of us refuse to read the last page or the last chapter until we know there’s another book available. It’s because we don’t want it to end, right? So if a writer hasn’t written to the end yet, accept that’s a good thing. Yes, even if they die before writing to the end. It leaves it open. And open means anything can happen that you can imagine.
So go imagine. Go do the work of writing yourself, even if it’s without a pen. Dream about what happens next. And let it change every time.