Your only point of argument is "the Third Reich could have survived to this day hidden inside the moon, as you cannot show me definite prove it hasn't. Hey, I said could have so you can't tell me how stupid my argument is".
Bro, you dunked on nothing, plenty of stuff points towards Joyboy being the last Nika user and having died 800 years ago, precisely why Zunesha was punished for being his companion, precisely why Zunesha hasn't heard the drums in 800 years, precisely why, after his dead, the Gorosei have been chasing the fruit for 800 years, and precisely why the Gorosei say "no one has awakened the fruit in centuries upon centuries", and precisely why Neptune straight out says "Joyboy is a person who lived in the surfaceduring the 100-year void".
And also, precisely why the One Piece is waiting "for Joyboy" to return, and the time hasn't come yet, as "Joyboy", aka Nika, aka Luffy, hasn't arrived yet, because there has not been another Nika yet.
What points towards Joyboy being alive for centuries after the void century, as you claimed? What base do your assumptions have? That slaves refer to the legend of "Nika" instead of the legend of "Joyboy"? Even if you were right about said legend being based on the actions of multiple individuals, wouldn't it make more sense for said individuals to have come before Joyboy instead of later, with all evidence pointing to Joyboy being the last one? Yes, it does.