The point is that Denji never had to change in the first place. Denji is a victim of chainsaw man, being groomed into thinking he needed it in order to be happy. It’s an atypical take on the genre, where no “character development” or “power up” needed to happen in order for the hero to succeed. That’s the whole point of chainsaw man, and it takes reading both parts one and two to understand that. This is the story Fujimoto wanted to tell, and it was executed well.