I still don't get what people expected out of this going in to be disappointed by the ending. it was pretty clearly steering in that direction even from the first arc - dark and light sides of humanity through times of extreme struggle and all that. The characters pretty much only existed to serve the narrative message so they're all a bit half-baked. The ending is kinda way too on-the-nose with the message though, like the author was worried people wouldn't "get it" after 88 chapters of the stuff, so they ended up hammering you over the head with it.
I guess maybe if you were in it for the sake of romance between the main characters, maybe that's grounds to be pissed? I dunno.
I liked this a lot more a few years ago than I do now, but it's still a really good series top to bottom. Successfully captures that specific, sickening feeling of an unavoidable natural disaster. Awesome art, too.