It is difficult to comprehend due to how different western and japanese market are. I don't think anything can challenge dragon ball and naruto in most of the world.
But every number I see about demon slayer, coming from japan, puts it as the biggest anime of this century, if not all time. Manga after season ending, home media, streaming and now box office. Only thing left is video games.
Manga is already pointed out. But in dvd/br sales, KnY did half as much sales in 1 year as one piece has done in it's whole lifetime of 20 years.
I think it's way more than good animation. Any japanese here who can explain the reasons?