Chainsaw Man Ending Spoilers!
A critical detail that is mentioned multiple times in Chainsaw Man is Pochita can't fully erase things. Stuff lingers. Emotions, memories, even tangible things like Hybrids. Pochita ate the Hybrid Devil but they still existed. Makima remembers devils, Public Safely knowing about ears. Denji knowing his catchphrase was missing something. Yoru feeling sad about Death. This stuff can't be erased fully.
Power just randomly decides to find Denj? She made a pact with him to always find each other and be best friends. Denji asked Asa to come to a new world with him. Aki wanted both of them to be safe and happy.
Nayuta is Nayuta like how Denji raised her.
The new world is the way it is due to the choices they made. Asa will move on due to Yoru's message to her about sins are nothing in the eyes of devil, Denji is free of his trauma from Makima- didn't even talk about sex this chapter at all. In fact he dreams about going on a lunch date and playing games with one. Aki is going to be happy with his family since the Gun Devil would never have a reason to attack, Himeno, Kishibi, Kobeni- none of them would be manipulated by Makima. Quanxi is off with her lesbian harem living her life.
Denji even makes a mention it ‘felt like he had a good and bad dream.’ He remembers the emotions and felt a spark when Asa calls him Chainsaw Man.
Pochita can't erase the bonds they make, and inevitably they may draw to each other.
Their struggle resulted in this better world.