Good observation! You got me thinking. (Disclaimer: I like Demon Slayer. It's a good manga.)
At first, I also agreed with you on this. But then I thought some more. And I realised, these are two different things, two different approaches and settings.
For Kimetsu no Yaiba, getting "decapitated" after learning their backstories showing how far gone they had been. Most of the characters getting killed or died this way were beyond salvation. Death was their release and/or relief of guilt.
For Dandadan, the characters are opposite of this. There's still something there for them to continue on, to mend things, and to be better.
I like both! (Manga, perspectives, philosophies, storytelling styles, arts, etc.) And I really like how your comment made me think! Thanks!