I think all this discussion about One Piece Vs. Kimetsu no Yaiba is a bit boring lol
Yes, Kimetsu no Yaiba won last year japanese sales, and of course will win this year. And this year will break One Piece 2011 record (probably Kimetsu no Yaiba will surpass 55 million copies, or even 60 million this year). Yes, Kimetsu no Yaiba is a HUGE success and I think it's good for japanese market that a new phenomenon had appeared, especially for all new readers that had arrived with its new success.
But I think Kimetsu no Yaiba is still far away from the importance or the legacy of One Piece in the Manga/Anime world. And when I say One Piece, I can say other series like Dragon Ball, Slam Dunk, Kochikame, Hokuto no Ken, Kenshin, Dr. Slump, Detective Conan, Mazinger, Evangelion, Gundam, Lupin, Jojo's Bizarre Adventure, Berserk, Ranma, Death Note...
Will Kimetsu no Yaiba be part of this list in the future? Yes, I think so but only time will tell.