Bingo. If no one is buying a series that sends out the wrong impression, not just to the writer, but to their publisher.

Both self-published and publisher-supported writers need sales figures. They’re a barometer of public interest.

If the books aren’t selling then it suggests they’re not popular, which can make a publisher think the writer themselves is bad, or that the story simply isn’t a draw for people, resulting in the withdraw of support.

For a self-published writer it’s often even worse. If the books aren’t selling then it suggests either the story isn’t grabbing people’s attention, or the market for that type of story has declined to the point that it’s no longer worth publishing. In that case the writer may give up on writing, or give up on publishing their story and never bother to share the rest of it with the world.

To get more personal for a moment, I’m a writer myself. I’ve self-published in the past. This isn’t my day job, it doesn’t pay the bills, it is a little side money from doing what I love. Even with it just being something I do for the love of it, actually putting together an ebook to publish is a lot of work. If a book doesn’t sell? I’m not going to go to the effort of making the sequel into an ebook. I need to know there’s readers interested in it if I’m going to go to all the trouble of getting it self-publish ready.

Ya but if a story is going to take 30 years to tell over the course of 6 books, that is asking quite a lot from the reader. Not only for the waiting period but also because they're likely to have to re-read to remember anything about the books.

That is to say, I do not care if they need us to buy the books to "keep on going", my time is more valuable to me than their time

thats exactly what I was thinking and wondering if already mentioned book in my post made maybe 1% or 10% of readers stop buying unfinished books. that sould hurt other authors would not it? but thing I am really interested if such a people are so few that it wont affect other authors, or is there high amount that other authors would feel it.

you get less income, you think less people carw about your book and your motivation drops. just because people are scared of starting new book since they forget first book before 2nd is released. or they are scared that they will get unfinished book. and they dont dare to read your book. otherwise author would have more readers

The lightbringer books have a nice recap at the start of each book

You can always buy the books as they come out and wait to read them until the series is done