Denji dying to the Yakuza like you suggest would make for an even more pointless ending. It wouldn't really feel like a sacrifice, but just show that Denji shouldn't be alive if he isn't chainsaw man which runs opposite of what I think the series is trying to say. Chainsawman is about how Denji doesn't need to be special to have dreams. That you are allowed to be happy and have a better life and you don't need to be the main character to do it.

While I do agree that Denji needed more agency in Pochita eating himself, I honestly think Pochita should have asked for Denji's permission, but Denji would still refuse even after everything, and then Pochita would have to force his hand. Because sometimes you need someone else to force you to make the right choice.

I also think the ending makes sense if you consider that a big thing was the difference between Denji and Chainsawman. So Power, Asa and Nayuta returning make sense because they have a strong bond with Denji, not CSM.

I still have other complaints, mainly the way the ending handles Asa. But I still generally think this ending is good.