I think the original 'Demon Slayer: Kimetsu no Yaiba' manga is the best. The story is engaging, the art is amazing, and the characters are really well-developed.
Some classic novels are overrated because they were written for a different audience. 'The Canterbury Tales' was written for a medieval audience with different values and interests. Today, some of the stories may seem crude or overly moralistic. The writing style, which was innovative for its time, can be a bit off - putting for modern readers who are used to more straightforward narrative styles. This disconnect between the original audience and the modern one can lead to the perception of overrating.
I think it's because some people expect too much from it. Maybe the hype around it sets unrealistic expectations and when it doesn't meet those, it gets labeled as overrated.
The Demon Slayer manga ended possibly because the author felt the story had reached a natural conclusion and all the major arcs and character developments were completed.
I think '50 Shades of Grey' was misclassified as a crime novel in some cases and is overrated. It has a weak plot that is more about a controlling relationship rather than a true crime - solving narrative. Also, 'Gone Girl' is sometimes overrated. While it had an interesting premise, the characters are so unlikable that it can be off - putting for many readers. It also tries too hard to be 'clever' with its plot twists.
There are a few novels that I can recommend to you. They are all stories about being reborn as a Demon Slayer. The first was "Rebirth of the Sea of Laws, I Slay Demons in the Human World" and "Rebirth of Slaying Demons, I Actually Became a Demon Venerable?" They all focused on slaying demons and told the story of the protagonist becoming a demon slayer after his rebirth. In addition, there was also "Xu Xian's Demon-slaying Daily Life (Demonic Meow)" and "Black Moonlight of the Rebirth of the Demon Lord," which also involved the demon-slaying plot. These novels all have different styles and storylines. You can choose to read them according to your preferences.