It isn't about only taking a single idea. It is the fact, that he continues to take multiple ideas. Every arc, he needs to google other peoples work and add it into his. Almost everything relevant in One Piece was a work of someone else that he took from. Joy Boy was taken from Carribean story. G5 is Tom and Jerry. Wano ending was Plato's Atlantis. All these are major plot points of One Piece, where all of them were ideas that he took. At some point, your ideas can't continue to be from other's work.
It is a bunch of standalone ideas and concepts placed into a new setting spread out over 1000+ chapters. Big woop. I am going to take a wild guess and say that Caribbean Joyboy probably never fought 7 Warlords, 4 Emperors etc etc?
Nobody has copyright on mythology and Tom and Jerry doesn't have a monopoly on cartoon physics. None of this is "someone else's work".
Are we also going to criticise Oda for having Giants? Fairy like creatures in the Tontatta Dwarfs? Beast folk? Centaurs and a harpy? Where does this end?
Look, if you want to see how taking inspiration from various real life cultures and putting them together with fantasy elements ends up with a new setting, look at the setting of Warhammer Fantasy.
There isn't a whole lot of "pure" originality in Warhammer Fantasy. It doesn't rely on that. It takes a boatload of inspiration from the real world, blends some cultures together from different time periods, adds some fantasy races and the interactions between all these factions ends up creating a new world with a new history that entertains people.