Which book series went on WAY past its expiration date?S

The best science fiction and fantasy books are so addictive, you finish one book and immediately want to reach for the next. Until... they start to go downhill. Because the Law of Diminishing Returns applies to every universe. What's the book series that most outstayed its welcome?

Please include a book cover or other image of your chosen series. And no personal attacks on authors, please!

Top image: Sisterhood of Dune by Brian Herbert and Kevin J. Anderson.

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