She's the author. There is no "should" or "shouldn't". It is as it is. Animorphs is what we got. If you've got an issue with it, that's what fan-fiction is for. I mean, yeah, K.A. was pretty harsh way back in the day with her retort to all of the whining, but you know what? Good for her. She's right in a lot of ways, the people who get so militant about the ending could be said to not really 'get' the series anyway, so more power to 'em. She explained her reasons, I'm sure you have yours for wanting it different. Fact is, though, Animorphs had an unhappy ending, and it seems pretty clear in hindsight she'd planned that for some time.
Ergo, a happy ending for Animorphs would no longer be Animorphs.
(Holy crap. I just said "ergo". What a smarmy smug a-hole. I dislike myself.)