Say what you will about the rest of part 2 but I do think the ending conceptually makes sense. In execution it came out of nowhere and definitely needed more time in the oven, but the idea of it seems just fine, imo.
The whole point is that ignorance is bliss. Denji was obviously still happy when he got to go on dates with Asa and had Nayuta in his life, for example, but all those things just caused him to worry more. Nayuta, just like Power and Aki, eventually got killed off and Asa was a little fucked over by the fact that an evil devil controlled half of her brain and wouldn't stop until she killed the devil that Denji is merged with. All that objectively put far more stress on Denji than if he had just lived by himself with Pochita in a little shack like before.
Not to mention, Pochita might believe that him being in Denji's life only caused Denji more pain. After all, most of the bad things that happened to Denji were due to Pochita merging with him (like Yoru constantly trying to kill them). With Pochita erased, even if it's just temporary and chainsaws are reinvented like nukes, he at least gets to live a truly "normal" life for once and for all.
It's a weird setup for the ending, and you don't have to like it, but I think it'd have been fine if Fujimoto only did more to prepare for this type of ending. It came too soon.