One Piece sales have been dropping since the New World began and the reason is obvious to see when reading the manga.
Oda can never deliver what he promises. After Marienford, expectations were soaring, which resulted in a huge sale on Fish Men's Island. In Punk Hazad it is already possible to see how One Piece did not fulfill the expectations of the New World.
It's possible to notice how One Piece sells more at the beginning of new arcs by the hype, but as the story progresses people get disappointed and drop One Piece.
The most notorious case is Whole Cake, which went on to sell 4 million copies per volume and ended up selling half. One Piece lost half of the audience it had at the time, and this is not unexpected as WCI is one of the worst written One Piece arcs.
Oda can never deliver what he promises. After Marienford, expectations were soaring, which resulted in a huge sale on Fish Men's Island. In Punk Hazad it is already possible to see how One Piece did not fulfill the expectations of the New World.
It's possible to notice how One Piece sells more at the beginning of new arcs by the hype, but as the story progresses people get disappointed and drop One Piece.
The most notorious case is Whole Cake, which went on to sell 4 million copies per volume and ended up selling half. One Piece lost half of the audience it had at the time, and this is not unexpected as WCI is one of the worst written One Piece arcs.
Most people started buying volumes during Marineford specifically
And so far WCI (a much better arc than Wano) has better sales than most if not all of Wano so far
Imo Wano just has a lot less to like than WCI so far and that’s why the sales continue to decline
Oda dropped the ball with the arc and most will acknowledge that but he still hasn’t moved on after 2-3 years