There’s two main ways a long series can decline, I’ve found.
The two are:
- Type 1: The author’s heart isn’t in it. It’s continued for financial or just sunk-cost thinking reasons, it might be farmed out to ghostwriters who only care about the paycheck, and there’s just less passion.
- Type 2: The author has become successful and/or confident enough that they can go hog-wild, any editorial or self-restraint goes out the window and the whole thing can turn into a vanity project the writer likes more than the readers.
Note these are not incompatible (author is tired so they make it more ridiculous to help themselves through it…), and Type 2 can shift into Type 1 pretty quickly.
Having read all 27 (!) books in Jerry Ahern’s Survivalist series, it devolved into Type 2 around the tenth or eleventh book once it kept going past a good stopping point and stayed there for the entire rest of the series.
Harry Potter got hit with Type 2 pretty hard after the third book, in my opinion, while a lot of mystery novels tend to become Type 1-as did Janet Evanovich, sadly. (I loved one of her early Stephanie Plum novels despite not being the target audience, but looking at a later one showed she’d lost her touch)