I got a little irritated with the way I saw people react to the ending of The Incredibles. As a refresher, the ending shows how the family of The Incredibles is getting along after the story's conflict, living out their everyday lives with renewed happiness and confidence. Then, a new villain known as the Underminer appears out of nowhere and delivers his monologue about how he's about to carry out some great evil plan. The Incredibles look at each other and grin as they put on their masks, and the movie ends.
And people seemed to be bothered by it, expecting that there had to be a sequel to resolve this ending. But that was an ending; it wasn't about what was going to happen next, it was about the tone that was being sent, emphasizing what the characters have been through and what they stand for.
Animorphs used the exact same ending format. The One is more or less the same as the Underminer, albeit with several more layers of meaning. The ending isn't about what's going to happen next, it's about who the characters are and what they stand for. Harry Potter got a softer ending because a major theme of that story was the significance of parenting, and the ending emphasized that theme. Animorphs was about "Live free or die." The ending was congruent with that.
K.A. gave Animorphs the ending that suited the series. Think about the experience of reading it: each chapter lasted only a few pages, and the vast majority of the chapters ended in a cliffhanger. Reading the books consisted of jumping from one cliffhanger to another. The dramatic cliffhanger was a staple of K.A.'s writing style. The ending was congruent with that.
The books were about the Animorphs being faced with some impossible situation, coming up with some insane plan, Rachel saying "Let's do it," and Marco saying "Are you INSANE?!!" The books end with the Animorphs aboard The Rachel, and Marco groaning as he recognizes Rachel's grin on Jake's face. The ending was congruent with the books that came before it.