Umm... about this:
Yes, I agree with "a little", like the One part.... but other than that, not much of a bad ending. I mean, no one is perfect, right? And you can't satisfy everyone either. 
I think it's safe to say that the vast majority of Animorphs fans weren't satisfied with the ending.
I can't agree with this. I don't see much people that weren't "satisfied" with the ending. not much. I mean, I don't see Estelore and morfowt fending of every member of RAF in the K.A defense debate LOL. Maybe there were some points a vast majority of the fans didn't like, but not necessarily "not satisfied".
I didn't like some points, but I am "satisfied". I like the ending. Sure, mighty morphing kids aren't realistic, but that doesn't mean their motives, actions, and thoughts should be unrealistic as well.
You have to consider this forum's target demographic. Everyone on here is a serious Animorphs fan.
Pretty much everyone has seen Star Wars, but only about 10% of them are serious fans who buy all of the merchandise and go on websites and spend a fortune on action figures they never take out of the box. Most people know Darth Vader, Chewbacca, and Emperor Palpatine, but only a few people could rattle off the names of every character you see for 3 seconds as the camera passes over them on the streets of Coruscant.
The average person isn't going to be satisfied with a Debbie Downer ending, regardless of the genre. I'd say that the only genres that unhappy endings typically work for are historical fiction (depending on the event) and love stories. And even then it depends on how it's done - The English Patient was much better than Becoming Jane.
Regardless of whether or not you thought it was justified, the average person who isn't an expert on the series isn't going to like it.
I have my own reasons for not liking it, but the fact that it would be so hard to sell just sticks out in my mind as well. I think that the books would've stayed around longer if it ended on a better note.
