In my opinion, the author has Paul die because that helps him to make his point about the effects of war.
As we have seen in Paul's thoughts, he feels that the war will destroy his whole generation. He does not mean this literally -- he doesn't think they will all die. But he thinks they will all be destroyed emotionally and spiritually.
Because the author felt this way, he has Paul and all his friends die. He is symbolizing the idea that the war has figuratively killed the entire generation.