Luffy is Joy Boy no matter how you slice it, and it’s not just inherited will, because he has his fruit, too, if not his specific demeanor, inclinations and possibly even,
yes, his spirit (devil fruits have a “mind of their own”.) We Don’t even know what Devil fruits are from Vegapunk yet so keep that in perspective.
It would be one thing if the Gomu Gomu was the Gomu Gomu and Luffy had just inherited Joy boys sense of adventure. But no, Luffy has inherited and possessed the Nika fruit which has the heart beat of Joy Boys liberation drums. He’s not just some rubber boy. He is a specific continuation of Joy Boy himself.
Fact of the matter is, Luffy is about to keep Joy Boy’s promise, as a measure of fate. You can call it what you want but he is the Joy Boy Kaido has been looking for, through no other means but so-called destiny.