Decades long series losing sales is nothing surprising. The audience they are directed to simply cannot easily get into it (and no, despite the editor's claims, Wano is not a good way to "get into One Piece", and no one is jumping into a series starting from the 90th volume), and the long time fans start losing interest in it as they grow up (or they just simply don't feel like the series is as good as it used to be).
If anything, One Piece holding up its sales for so long, and even managing to reach its peak after more than a decade in the running was the surprising part.
Long running series can still be appealing to the new generations (although with Japan's declining birth rates they are never going to reach OP's heights), but not 20 years in the running series. Kimetsu being a short investment helps it being picked up by all sorts of demographic, some of which would normally find a potential 10+ years investment too intimidating.
As for potential claims of "OP only losing to Kimetsu due to corona", considering Kimetsu has already outsold any 2017-2019 full year OP volume before the half year mark, I wouldn't exactly give them any credit.