The ending of CSM echoes and answers the town mouse vs country mouse problem

Throughout CSM, Denjis life with Chainsaw man is like the town mouse, more chaotic and dangerous but more fulfilling and interesting

Whereas Denjis life without Chainsaw man is like the country mouse, safer and less chaotic but less fulfilling and less interesting

Throughout part 2, denji struggles to pick between these two options, with him wanting both options despite him only being allowed to pick one. Although we do see that when Denji chooses the town mouse (Chainsaw man), his life gets significantly worse and he starts to lose those around him and any chance of going back to the country mouse. Because of this, Pochita has to step in and decides that Denji should live life as the country mouse, without Chainsaw man, rejecting the idea of the town mouse as it is inherently harmful to Denji. It also had to be Pochita to do this as Denji was already too far gone and had removed all chances of returning to the country mouse hinself due to his addiction to Chainsaw man and the chaos and deaths that had caused.

I think that one of the main messages of Chainsaw man is that the country mouse is the better option for Denji, with this even being said by Reze back in her arc (meaning that her character wasn't made "meaningless" by the ending) and the ending proves it.

Overall, the ending makes perfect sense given the events of the story and was even foreshadowed back in the Reze arc. Praise Tatsuki Fujimoto.