I know Animorphs is known for it's grim ending, but on reread the ending leans far more toward "and the adventure continues" than it does "rocks fall; everyone dies."
The One and Two Years later portions are entirely about setting up what Earth's new normal is in the universe. Marco and Jake are both right in their descriptions of everything happening in stride and also tensions being on the rise. It's a new world and that means there's endless possibilities.
The One Who Is Many encompasses this to: A new threat exists. But it's not just a new danger. It's also a mystery. Why does it call out Jake in particular? What does it actually want?
Even "Ram the Blade ship." isn't as dark as I remembered it being. I believe the authors have said they don't even agree on whether the crew of The Rachel end up dead from this move or not. And as far as the tone is concerned, I don't think it matters. Weird as it may sound either option leads to a feeling of there being more to know: it's just no longer in the scope of this story.
As a kid I was honestly more mad that it was a cliffhanger than because the crew of The Rachel probably died. I wanted more, dammit, and the book was refusing to provide it. Over time I ended up convinced it was "grim" but honestly? It was more like that scene in Avatar: The Last Airbender when Zuko gets to the end of the scroll and says "That can't be it. Where's the rest of it?"
It's sort of like the ending to the first book where the feeling was that the story wasn't done yet.
Not that it needs an official sequel or anything. As a story it's complete. Highlights and flaws it's a full story with an ending that tells you the world will continue to change.
There's a reason this book is called The Beginning and everything about the last book feels like an opening to something else even as the current story closes. It's dark in many ways as we see Jake and Tobias and Marco's dissatisfaction with the post-war but it's also fiercely optimistic in just about every other way.
It's why I can never get this series out of my head.
The more I think about it the more the ending really is so very in line with every other "ending" that came before it.
The Andalite Chronicles ends with Elfangor's story but begins that of the Animorphs.
The Hork-Bajir Chronicles ends with Jara telling Tobias that stories don't end.
Even The Ellimist Chronicles is about a beginning at the end of Toomin's world.
Everything continues. It may be dark optimism but it's optimism nonetheless. The tone isn't any different from what's come before it. It carries on the same theme.
I agree — it feels like that book fulfills the promise in its title! It's not the end of the story; it's the beginning of a new one, in this new world that the protagonists created.
