Series fiction and jumping the shark

Does series fiction inevitably run out of steam?

Sarah Weinman points to this article about the marketing machine that is Janet Evanovich and adds,

[It] doesn't really get to what's likely the bottom line: that as the marketing hoohah has increased (and so too have her sales) the quality of the books have dropped off rather sharply.

But then, does it really matter when the publicity works so well?

I've only made it through number nine of the Stephanie Plum series, To The Nines and I'll probably read the two most recent installments eventually, but I noticed a drop off in quality a while ago. They're still enjoyable, mind you. Just not as much as before.

I didn't grow up in New Jersey, but I lived there for a long time and I know people who grew up in the milieu Evanovich depicts: Ethnic working class types who still live within a few blocks of where they grew up--if not still at home. Evanovich did a great job making the world of the burg, the neighborhood of Trenton where Stephanie lives, come alive. She also showed a great affection for the place and portrayed the characters without condescension.

The first few books also had this great antic quality, as our herione zipped along from one embarrassing incident to another. But as the series progressed, the antic quality became forced, more high strung than hilarious.

So I wonder, is it possible to sustain a series like this for any length of time? Or is it inevitable that such series will jump the shark like their TV counterparts?