I'm gunna be honest, I didn't like that ending either.
EDIT: In reflection, I actually do prefer the original, it goes out with a bang. This one kinda reminds me of Land of the Dead where they just let the head zombie go and just say "He's just looking for a place to go".
Yeah, I agree. It was certainly "happier," but "happier" does not mean "better."
I feel like this movie lacked a certain je ne sais quoi, but I know that this wasn't it. From the original ending, I got the impression that they couldn't be saved, and I thought that was interesting because not everyone can be saved all the time. Sometimes, they need to be killed.
This ending gives me the impression that all he did was for nothing. I liked that the monsters still had some sort of human aspect in them, but this ending makes no sense unless the parts of the movie is redone because the rest of the movie plays like they're cold hearted and have no soul. Here, it shows that if they were left alone, they'd be fine. That's cool and all, I guess, but that really seems a little anticlimactic.
In retrospect, this ending definitely would have made the film be more well received, but I appreciate the original ending more after watching this. It wasn't the cliche, typical happy ending that was expected. Unfortunately, happy endings are why people go to the movies, so there you go.