Why?
Yes,
Kimetsu no Yaiba success and sales are
AMAZING but I think a lot of people underestimates all
One Piece has done in the last 10-12 years.
I think Kimetsu no Yaiba 2020 sales will surpass One Piece 2011 record (probably Kimetsu sales will be over 40 million copies in 2020) but we must see it in perspective.
Do you remember One Piece's situation in 2011? One Piece had a backlog of 60 Volumes and 200,000,000 copies printed. I mean, One Piece sold almost 38,000,000 copies with a huge consolidated fandom and a huge backlog, and even in that situation, One Piece could gain a lot a new readers in those years (2010-2011-2012) thanks to Strong World Film and Marineford arc in Anime.
Kimetsu no Yaiba situation was very diferent. When boost started last year, Kimetsu had a backlog of 14-15 Volumes and a very modest sales. Thanks to the Anime popularity and social networks impact, a lot of new readers arrived and bought all Volumes to catch up with the story (that's why all Kimetsu Volumes has similar total sales).
I don't know what do you think about it but for me, even when both cases are
AMAZING, I think it's more "easy" get an extended boost for a few months/years in Kimetsu situation (or Shingeki no Kyojin, Death Note, Full Metal Alchemist... in the past) that in One Piece situation. I mean, for a new reader it's much more easy start a new Manga with only 15-20 Volumes on sale that another one with 50-60 Volumes