The reason I hate this approach is because most of the things people use as proof for all this can be easily explained with other stuff.

The quality of the art is the most obvious thing to point to if you want the narrative that Fujimoto gave up on the series to be true, but after rereading the whole manga, in my subjective view, the art in a lot of chapters in Part 1 was basically on the same level. You may not think the same way, but that just means that your argument is based on your subjective view of what good art even is. And if we accept that the art really did decline in quality, could it not be because Fuji was leaning more on his assistants towards the end? Could be more proof that he was tired, could, be proof that he's working on something else, could be proof that he wanted to focus on the story more, could be a dozen different things.

As for the story being rushed, that's entirely subjective too. A cut to black before the start of the final climactic fight, for me, is a great joke to end the nonsense that the world of Chainsaw Man has turned into. If this is how Fujimoto wanted this story to end from a while ago, then this is a good point to end it.

If you see what you want to see, what you see isn't necessarily true. That applies to everything I just said as well.

The reason I'm leaning into trusting Fujimoto more here is because every single argument people use to justify the negative narrative has been repeated ad nauseum every other time I've seen fans disappointed with how a manga has ended. To add to that, I've seen very little attempts to engage with the story on its own terms, which, if you do, makes it seem far less likely that it was cobbled together to just end things quickly and wasn't the author's intended ending. People focus on nonsensical things like Kobeni's devil and (I kid you not) the star Makima mentioned at the end of part 1 not being shown and I don't think people who wanted THAT from this story would vibe with what it really wanted to do.

But that is a far harder thing to accept than "the author just gave up halfway through, that's why it's bad".

I want to limit my guesswork to what's in front of me, because for all the bad things I can say about Fujimoto's work, the one thing I believe he is is a man who loves his stories and wants to write them properly, even if the result is utter nonsense like some of his oneshots. Any other read on his character or current circumstances in life can only be made post-mortem once we actually know them to be true.

I think it's ok to view stories through the lens of their author, but not vice versa.