I wasn't saying it was the only important thing, just arguably the most.
Having said that, I'm certain KnY would have beaten OP's lifetime sales if it ran for a few more years. Especially if the movie boosts it further.
This assumes it maintains it current popularity which is impossible, one piece exploded in 2009-2012 but after that it returned to normal and is declining right know.
You forget that most long running series experience decline and if the author extends the story past it natural end the decline would be harder(for exampel if after luffy found the one piece Oda conteniue the manga the sales would drop)
I understand your point, but I answer it is because it really is the only argument that some One Piece fans grab - since it is the only record that Demon Dlayer has failed to beat, and never will. But DS reminds me a lot of FMA. You don't need to have more millions than other manga for your work to be the best one. Be considered a cultural thing because it marked an era. A clear example is the FMA anime. Over the years I have systematically seen fans choose FMA as the best anime ever. Obviously it is somewhat subjective, but in general you see that the majority choose the same anime, that means that said mangaka and studios are completely calm. Because they have accomplished a job that few have accomplished.
Is Oda panicking? I don't get it?
Oda hadn't spoken against demon slayer nor was he telling people to go buy more volumes.
Also the one piece anime sucks, it adpats one chapter per episode for god sakes.
Every opinion is subjective for some people black clover is better than one piece, demon slayer, naruto...
For others it's fairy tail.
For others it's hunter x hunter or berserk or death note or the world's end harem(yes there are people that claim this) ....
You get the point there is no objectively superior manga because if we are objective about manga we wouldn't be able to decide which one is superior because that would require us to put objective standards to quality and thats impossible.