I finished rereading a much loved book from my middle school/high school years. It's book 3 in a seven book series (and no, it's not Harry Potter, I'm not that young), and the series had been designed from the start to be exactly seven books. None of this extending the series because it's popular or because each novel is a stand alone deal. It was intended to be seven books. No more, no less.
I really liked the third book because it took the series to another level. It was the dark turning point where the stakes were raised and ancestral enemies realize they might not be enemies at all. And I fell in love with one of the periphery characters introduced in the book. He had to grow up fast and became a tragic character by the end. I think I may have cried the first time I read the ending.
After that, I was very bright-eyed and optimistic about the series, and while books 4 and 5 never quite peaked as high as book 3 with me, they were solid. Then something happened with books 6 and 7.
Without going into specifics, the spelling of my favorite character's name in book 3 was changed by one letter. Not much, but it certainly threw me off. The prophecy from book 3 never came back again (despite being a huge deal in that book) and my favorite character didn't do much of anything except act like a talking piece of furniture (and he'd been the one the prophecy was about).
There were other issues too. Some minor. Some big enough that one could drive a truck through. The implied gender of a character's child was swapped between books. And even within the same book an implausibility happens that makes sense on an initial read (while the main characters and the reader don't know better), but fails once a certain character is revealed. Astute readers will catch it on the first read and check back a few chapters to see if the events leading up to the reveal make still sense. I remember I did (and they don't).
I still appreciate the series for its characters and fine world building, but finishing it bothered me as a high school reader because I thought someone would have planned the whole thing out from start to finish, since it was known from the beginning that it would be a seven book series. I felt like the tail end of the series had been phoned in, nobody cared anymore. Or, the series really had not been planned as well as I thought it was, and so the series had been wrapped up as neatly as possible given the circumstances.
There's probably a reason behind the ending sagging that I'm not aware of. But as that high school kid, I was really disappointed. I was promised awesome sauce all the way up through the fifth book, and I got middling sauce by the end. It wasn't bad, but it wasn't good, and felt worse because it wasn't awesome.
When I think of the most popular seven book series of today, when I think of Harry Potter, I realize that whatever I may think of Rowling's prose, I respect her ability to write a plot. She might have added and subtracted things behind the scenes that the reader never knew, but what was laid down made sense and I didn't feel any promises betrayed.
I've never written a long book series, but this is something I want to keep in mind for when or if I do.