"Joy Boy" is not a singular person, but rather a state of being.
That's why Kaido believed he could be Joy Boy. That's why Kaido, upon finding out he couldn't become Joy Boy, was looking for someone who could become Joy Boy.
Luffy 99.9 percent most likely achieved this due to the way he lives his life. He just wants to be the freest person on the sea, partying, and having a good time with everyone he meets, and that is most likely what it means to be Joy Boy.
If you've been reading One Piece all these years and don't think there's a deeper meaning to all of this, outside of "reincarnation", then you've been following the series wrong all this time.