We can see Dressrosa did very good till the end (vol 70-80) even tho a lot of people were trashing the arc, the decline started with Zou arc (vol 80-82) and Wano so far has even more sad numbers in comparison even to WCI. Idk what is the factor, I'm not Japanese fan and idk why they buy OP two times less then they used to, but for me, personally, Wano arc isn't that good because I can't bring myself to care about the scabbards and the other samurai, who have the most paneling in Wano (safe from Zoro and Luffy.) They're good characters probably but I just... don't care.