Seems like OP dont attract the young generation anymore, they are much more likely to read tbe new wave of "horror shonens", that have been growing as a trend since Shingeki and Tokyo Ghoul.
I understand them, even if i love OP, i get that for them the themes of the manga dont have that much appeal like more edgy like things Kimetsu, Jujutsu and Shingeki have.
The seasonal animes with amazing animation, also help to boost popularity, different from one with 900ep from before a big part of the public were even born, that have a very slow pacing and a bad animation.
And of course the autor, that even if he is still a genious, is lost in those ideas of trying to make everything bigger, while a SH vs RHP Arc would have infinite more hype than Dressrosa/WCI/Wano full of shit side characters, he keep with adding more useless plots and characters to a manga that already have some amazing ones. As we can see that the SH fights, Oden FB and the inter-act chapters were the real good chapters of the Arc, but Oda keep focusing ob shit parts.
Another thing i would like to know is if It is just the fisical volumes, because a lot of people buy digital ones, expecially if u are going to need to buy 100 volumes. I myself stopped to buy because of the economic crises of the pandemic, and as Viz is doing a Brazillian version, i dont intend to start buying again.
Someone have the numbers for worldwide Sales?