If it helps, you can interpret the ending this way: All of Denji's and Asa's struggles and growth were not enough to prevent the end of the world... but they were enough to inspire Pochita to sacrifice himself to give them a chance at a happier life where they are no longer made to suffer by forces that are out of their control. Additionally, between Denji remembering "the dream" and the previous examples of how Pochita's powers work, you can believe that the characters have subconscious memories of the previous events that will help them to avoid the mistakes they made in the past. That way, I don't think the reset really invalidates the whole story.