To start off, I just want to say that I haven't read every word that's been posted in this thread because frankly, I don't have that much time anymore. However, I have seen some good points on both sides of the fence. Now on to my take on the subject.
I enjoyed the ending in a certain fashion. As someone who absolutely loves Animorphs of course I wanted to see more and to know more, but I can completely understand the ending and I'll lay out some points as to why it's believable and ok with me.
1) I think that Animorphs is definitely a war story. It's about people who are thrown together to fight a war that they didn't ask to be a part of. In some cases they didn't even really like each other. (Think about how Marco thought Tobias was weird and also his strained at times friendship with Rachel) These characters came together and bonded in a huge way through their experiences fighting the Yeerks. That's a war story.
2) To those who say that the spirit or message changed, I both agree and disagree with this. Any story that goes on for so long has to change in some ways. If nothing in the story changes, then you end up with static, one dimensional, boring characters and story lines. After so many years of fighting a guerrilla war, the Animorphs are worn down, they're tired, they're willing to take chances. Just look at the Auxiliary Animorphs. You really think that the Animorphs would have taken that risk at any point before that? Even if the Yeerks had made the kind of advances that they do in #54, the Animorphs would not have been desperate enough to use the blue box again so close after the David incident. The point is that they changed. They aren’t the same people that they were in book #1 or even the same people that they were in book #26.
Take a look back at an old picture of yourself, think about the person you were then, even one year ago, and I’ll bet that you can see a bunch of differences in who you are and who you were, and you weren’t involved in a crazy war with aliens.
3) In regards to the ending itself, I don’t think that it betrayed any of the characters. All of them take something different from the war. I don’t know if you can really say that any of them are happy though. Even Marco with all of his TV deals and everything still reminisces about his days with the Animorphs and morphs sometimes to get back to it. And once the Kelbrid come into the picture, that feeling of “it’s up to us to stop it” comes right back into play. The same feeling is still alive. In their own ways, I think that everybody wanted to be there. Not in an overt sense, but still, there is some part of each of them that wants to put them back in those situations. Plus, the story isn’t really over. The Yeerks were kicked off Earth but they still a big force throughout the galaxy. Don’t forget, the Yeerk invasion of Earth wasn’t even a full operation, at least for most of the series, it did pick up at the end obviously.
My point is that there is still a lot happening. The war is far from over. I equate it to Star Wars. If you follow the Expanded Universe and read those books, you find out that yes, the Emperor was killed at the Death Star II, but the Rebels/New Republic don’t attain peace with the Empire until 15 years later.
4) As far as Lord of the Rings goes, up until the end of the book, you have only 1 ½ deaths. I’m only counting main fellowship characters. I say half because Gandalf dies but comes back although Frodo and Sam didn’t know that. So the only real death is Boromir, and he was corrupted by the ring. Everyone who dies in LOTR was corrupted in some way. Theoden, Denethor, they were corrupted. So nobody “good” dies in Lord of the Rings. Even Frodo was corrupted and he essentially ended his life by going to the Grey Havens. And if you look at Tolkien, you can see that he wanted his characters all to live because of the devastation that he went through in World War I.
5) I think that the fact that all of us can feel these sorts of emotions about the ending is the entire point. We all fell in love with these characters and followed them for so long. They became our friends, our comrades, and the feelings that we get are exactly what those characters are feeling. So when you feel those sorts of things, it’s another emotional tie that the story itself creates for its readers.
Anyways, I could probably go on for a very long time about all this so I’ll cut it short here for now at least.