I don't think it's a worthy question to pursue, because the coronavirus just completely wrecked most industries.
....Uh...no?
I'm saying this isn't a normal situation, and it's bound to have ramifications beyond what we're able to directly analyze.
My issue with this is that we don’t really know how big of an effect that COVID-19 is having on manga sales. The manga isn’t stopping Japan from it’s Demon Slayer frenzy, nor is it stopping manga like Dr. Stone from increasing. And for volumes of series that are already declining in sales, the decline doesn’t seem to be sharp enough to really show a clear effect from the virus.
... One Piece didn't lose 50% of it's sales in 2020, it lost ~100k from one volume to another. Not that it's definitely because of coronavirus, especially considering it's the same decline it showed previously and the shoseki estimates being given are still around the same numbers for the same spots, but it's not absurd that some series (not only One Piece) did lose some sales (or they transferred to digital). Off the top of my head, Conan is another series that lost a good amount of copies in the first week this recent volume. Again, it might just be the natural progression, but it's not absurd to think sales slowed down. Even if stuff like Kimetsu and Dr. Stone is growing, maybe it would have sold even more, who knows?
Yeah, you lose the argument when all you have it "what ifs" and "maybes". And simpler answer for both One Piece and Conan is that people aren't as interested in catching up with series of such size. Both also release digital volumes weeks after the physicals, so you can't argue for digital either.
I think i've seen pictures of the orignal issue in which Jojo debuted in (Issue 1-2 of 1987) were we could see the 第一部 (Part 1) notation. So I think the part structure was planned all along
...No? Araki has said he did the shift mostly because he found Johnathan too limited a protagonist, hence why Joseph was such a radical departure.
I'm not arguing for digital for One Piece or for Conan, those were just examples. I'm just saying it's not unlikely that there are series that lost sales, @Koni even mentioned examples in WSC. Also, Conan lost like 50k first week, I think it's a sharp enough decline that it's not easily dismissable. You said "literally no other series", but we already mentioned series that had declines, so...
wow, somehow you are worst than OPfanatic.. how do you get point 2 come from???
I more meant series with chapter counts similar to One Piece.
Oh, I was sorta mistaken about Conan, then. Sorry about that. I knew that it was declining, but out of the top of my head it was way less pronounced than 50k a week, that's why I mentioned it.