Is that why One Piece sales are down?
OP sales have been stable for a while .
If you just looking at physical numbers well you not getting the full data .
physical manga sales have been down a lot in last 5 years while DD have been growing at a insane pace in Japan.
Which the place where we get most sale numbers from .
EDIT to show how insane OP sales still are it got to 500 million sales in aug 2022 in 3 more year it sold another 100 million copies .
If you look at orcion chart that people like to use that outdated now it have OP only selling 15 million in 3 years in Japan .
Which would be higher if you count DD.