I'd agree, but it feels to me like she lost interest after the David trilogy. Maybe started to lose interest before. Think about it; after the trilogy, you have an obligatory-feeling "Tobias finds out about Elfangor" book, a Helmacron book (not centered around Yeerks), a ghostwritten book, a Howler book (again, ignoring the Yeerks), and then the rest of the series is ghostwritten except for "good Rachel bad Rachel" and the final two books. And the Megamorphs and Chronicles, I suppose, but take into account that none of them except for MM1 actually deal with the Animorphs fighting the Yeerks in the actual war, and it really does feel like Katherine and Michael were just getting bored with the whole concept of the series.
I feel off topic now...