What is a difference between Luffy and Nightmare Luffy? Apply that answer to your own question.
If you think this is a good reasoning, it isn't.
Nightmare Luffy is a situational form; Joy Boy seems to be Luffy's fate changing him forever because he himself is the new Joy Boy. Even if we end up seeing a reincarnation of the original Joy Boy possessing Luffy's flesh and defeating Kaido, it hasn't happened yet if it ever does, so assuming you were right on Kaido winning is quite bold on your part.
I have a question for you: Sun Wukong became a different person after reaching Buddhahood? That's the mindset you may have to follow in order to properly understand Luffy's transformation.