Oh, of course there's that too. Including destiny definitely changes how the journey will be viewed but I was looking solely at this reveal of his true fruit and what it changes and I kind of feel like Oda atleast had some previous chapters showing destiny so he fumbled (for those against it) that a while ago. Still disappointing that it doesn't come off as Luffy struggling towards his goal, unsure if he'll really make it but more of him doing what he was fated to do (still puts in work but there's no lack of assurance that he'll complete his goal). Like how Blackbeard was told to his face by Whitebeard he's not the one Roger's waiting for or the prophecy regarding which year the person succeeding Roger will be born. Kinda makes it less enticing but in regards to solely the true name of Luffy's fruit, I think there are still good aspects to it.
"Destiny" as a concept does not really exist. Nor does it contradict a character "struggling to reach his dream".
Just think, why was it even Luffy's dream to become the pirate king?
Well, what's a fact is that the one piece world needs someone who finds the one piece and challenges the world government. This can only be done by the "pirate king"(a title the world government made up)
Luffy is the next Joyboy because he will be the same to the world, play the same role, do tue same for the people as the historical Joyboy.
"Nika" is just a name for now, and we don't know who invented this name. It might have been the world government or even the ancient kingdom.
Luffy will keep calling his attacks "gomu gomu no...." and won't call himself Joyboy either.
He is Luffy and people only
call him the next Joyboy like they call Zoro the next Ryuma.
Of course Zoro is not Ryuma. He doesnmt have Ryuma's sword currently yet people call him the second Ryuma.
His role as such is neither a direct result of his bloodline nor is Ryuma's sword imposing its will on Zoro or anything like that.
And all the same, the devil fruit is not all that's necessary to make Luffy Joyboy.
And: Roger was only waiting for someone because he got that info on Laugh tale.
There might not even be a "prophecy" in the traditional sense.
Joyboy probably had voat and possibly future sight. He might have just been capable of looking that far in the future where he saw someone being born 800 years later to fulfill what he couldn't do.