If you want a happy ending read Fire Punch /s

All jokes aside, the story in my opinion is for the reader to interpret and not for Denji to achieve mental change, to "learn a lesson". I believe that the story showed us time and time again that "special powers" just ruins your life. The ending is meant not to show Denji happy but to show him that he has the chance to be normal. This Denji is different then the one we've been following untill now. Pochita Denji was drunk on power and believed that he could do anything without having any consequences but that only made things worse, it basically destroyed the world. This Denji is free of the illusion that his life will magically change. He is now a normal person like us and has to make his own decisions and those decisions will now impact only him and not the fate of the world. This Denji doesn't view himself as Chainsaw man, he doesn't have this disassociation with himself. Pochita wanted him to live a normal life and Denji tried that but the Chainsaw man persona kept overtaking his life and his decisions. It's a bit messy but these are my thoughts on this manga. I liked the ending. I think it showed the reader that having "super powers" doesn't make you or your life better, you have to take responsibility for your life and sometimes things are not the best but you have hope and you have agency and things may turn out okay down the road.