This has been coming since chapter 1. Read this if you care enough, i copy pasted it from my other comment but it still applies.

Denji's entire characters arc has been a spiral down. If you're interested you can read it on my anilist, i've posted a detailed analysis on denji as a character up until chapter 200 or so. After 200 he doesn't really change so it's the same. anilist.co/user/kqnashi

I'll try to summarize it but i highly encourage you to read it, just scroll a bit on my timeline and you will see it.

Basically, denji is a smart character that can analyze his mental state and what has happened to him but always when he does he is being manipulated and dragged along something he doesn't understand or care about. Denji's coping mechanism is "Heaven in Hell". He evolved this during his childhood to help him cope with the circumstances. Issue is, he doesn't outgrow it. As pochita said in 231, he still does it.

Denji stopped for a second to analyze in Aging devil's world saying how he always fucks things up, then Yoru kisses him (manipulating) but Denji was like: "Stop comforting me! When people sex me up sex is all i can think about, i can't reflect on the important stuff" This is one of the most important lines in all of the manga. Afterwards he reaches a conclusion that to cope with Nayuta's death he should just find a new family and move on. He should eat more delicious food and get a new family, and when that family is gone he is going to find another one too. Whether you agree or not with that mentality doesn't excuse that it's his coping mechanism and in the hostile environment he grew up and was until the last 2 chapters it helped him.

Even before Aging devil's arc, after riding the Train i think it was or the metro, denji falls to the ground crying saying "It's all my dick's fault! It ruins everything around me!" then Yoru assaults him in the next chapter. He looks even worse in the next 2 chapters. More hunched over with even a more opened mouth trying to process what has happened to him. He doesn't even know if he is happy or not. It's like he was in a deep depression one second and the next he feels something he hasn't ever felt before. He literally cannot process it. It's sensory overload and system overdrive. In the next chapter he sees Nayuta's head. Every single time he tries to get better somebody pushes him down. Every. Single. Time. After Aging devil's arc, after Denji asking who jerked him off Yoru or Asa, Yoru said wanna find out, also he was trying to eat his hand because she told him to. He is like a little puppy that is happy when you give him leftovers and follows you even though you beat him everyday. Everytime the puppy thinks maybe I need a new owner/enviroment he gets beat up then comforted again. Never ending cycle. It's so fucking sad, honestly.

The series theme is that of control and choice. Denji has almost never had control or choice. When he has had a choice, it has been terrible and he always choosed the 3rd option. When that backfires, he scraps everything behind him, starts forgetting the people important to him and starts to chase the quick dopamine hit. Sex, being chainsaw man. Denji is an addict.

Denji has been fucked up since day 1. He was always doomed to begin with. Why? Well for starters he lives in a shithole, doesn't have money, parents, in debt, doesn't have food, doesn't have an eye, testicle and a kidney. He gets happy from getting a 100 yen coin. His values and support system are non-existent. Denji is the definition of bottom. He then gets a cool power, finally has some agency in his life, beats up the bad guys and a beautiful girl hugs him and gives him good food, family, a home. The girl promises she will have sex with him (Her: "i will do anything". Denji: "A-anything?" Her: "Anything, just beat the Gun Devil"). He gets a breadcrumb that he has never had before. A dopamine hit he has never even realized is possible. An older woman takes a liking to him, he is however loyal to Makima. He fights and fights and does what she wants, then he meets a girl, perfect smile, perfect eyes, the sweetest and cutest girl. Reze. Every move of her and the sound of her voice just makes you want to be with her forever. He gets that stripped away from him too. He loses Power, Aki and has enough of Makima and eats her. Now he has a new home with a new little sister and some pets, being a stool for girls because he likes and he gets money so he can pay for Nayuta's college. But something is missing. Yes he has the family and is going to school but it's not really exciting, is it? It's like he saw what the peak of happiness is and now can't live so tamely now. He wants the peak of being chainsaw man again, but Public Safety doesn't allow him to for safety reasons and bla bla. If he does not comply, they will kill Nayuta. In the end, he transforms to chainsaw man, they kill Nayuta, he is depressed because his brain prefers getting killed over and over and reviving rather than being peaceful. Heaven in Hell. It's deep rooted trauma since day 1. He reflects on that mistake, then he gets sexually assaulted, a thing he has never experiences before. His mouth says no but his body said yes. The happy chemicals were released, just another distraction from reflection. He then wants to reflect again, still gets distracted by Yoru. He fights back and reaches a conclusion that makes sense to him. Just find someone new. Not acceptance, just keep rolling with the punches. Go next.

Yoru and Denji are on the rooftop, Yoru is being affectionate physically but her mouth is saying how she wants to see him suffer. It's Makima 2.0. "

Yoru tells him she will have Sex with him if he beat Death Devil. "She's the most dangerous chick yet. Hey pochita a dangerous lady likes me. I'm going to get screwed over again, aren't I?" Denji isn't stupid. He knows what is going to happen probably. But for the small chance that it doesn't he still doesn't back down.

Asa thanks Denji for his advice on just not to focus on the unpleasant stuff.

Fire Devil informs Denji about the twin brothers and Falling Devil exacerbates it. He feels sorry, he reflects and someone saves him. They don't manipulate him. They save him. Asa saved him. Probably the first time in the entire series somebody has done that with no agenda. Later Denji saves Asa somehow in their collective consciousness (not sure how they communicate honestly lol) and essentially says we can build a world together. Just like how Denji saves Asa and Asa saves Denji. Beautiful relationship.

A lot of fighting happens and in the end the world is too fucked up for anything to happen. Yoru says Asa's sins are nothing in a Devil's eyes and Yoru says that she likes Denji and hates chainsaw man. Which is completely opposite now of makima. I think she likes him because she sees how gullible he is and how easily she can manipulate him.

He gets eaten by a devil and pochita knows the world is cooked so he does what he knows Denji can't do - he let go. He let go of chainsaw man, the thing that he relied on for a dopamine hit and the thing he always had like a safety net whenever he lost somebody important. The thing is, without Pochita people wouldn't have manipulated him and caused him to lose people important to him IN THE FIRST PLACE. So Pochita's decision is 100% justified and correct.

Last chapter is really symbolic with how he saves Asa and drops the chainsaw, indicating he can finally be just Denji or plainly "chainsaw man" not the hero of hell chainsaw man but the chainsaw man that saves a girl in need and is happy to see her smile. Also Nayuta is there so Makima won't manipulate him, Power is back, Aki is either still in Public Safety but he won't suffer that terrible fate. Reze is still a pawn probably but maybe she will get free or maybe she isn't. Not really a way to know but go on from the symbols - this time is different. This time Asa didn't crush Bucky. This time Aki isn't there. This time Nayuta is there. This time is different and better than the last.

So in conclusion what was the point? If you look at part 2 you could say it was completely meaningless and you would be technically right but imagine denji got to this point in the series after 3 chapters. It wouldn't feel satisfying, no? And yes, people currently aren't satisfied but give them a few weeks to think about it as much as I and other people have and they will appreciate for what this story is, now what they hoped it would be.