One Piece Sales are Down because it has only 2 volumes this year. Other series like Demon Slayer, Kingdom, etc. have 4-6.
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Of course sales will decline over time because One Piece reached its peak hype around Marineford. The hype will slowly die down for any long-running series. According to Google Trends, worldwide One Piece popularity remained consistent between 2010-2016. It dipped 35 percent between 2016-2019 and has rebounded a bit in 2020(like 5 percent).