This theory can be introduced with one simple question: What does it mean to be Joy Boy?
According to my theory, Joy Boy is the title for the person whose hardships did not kill their dreams, which means that they were able to get through with it and to bring something that changed the world entirely.
Why does it matter? Because Luffy will be Joy Boy, and he already has met a few contenders who failed.
1) Luffy’s Joy Boy-failed former opponents
While Oda’s repeated fake-out deaths are not particularly appreciated and very debated amongst the fan community, One Piece’s definition of real tragedy isn’t death, but the loss of hopes and dreams.
This had been made very clear in Skypeia, with Enel’s words as shown above.
In this case, Luffy has found on his way many antagonists who simply lost their dreams and hopes, and actively worked to destroy those of the future dreamers.
- Captain Kuro
Captain Kuro, who posed as Klahadore in Syrup Village, was the first antagonist to show signs of disillusion regarding his former objectives.
We could debate that Buggy may have been in the same case; but in my opinion, he never lost his dreams, as he always had the objective of having many riches in his possession.
Captain Kuro, however, was a pirate who chose to leave his freedom behind because he grew tired of that life. As such, he was as far as one could be from Luffy, or, according to this arc’s themes, from Usopp.
Deeply rooted in his belief that a calm and hidden life in an East Blue village would be more suited to his objectives, he abandoned his crew and eventually betrayed them, plotting to kill them all as they would come to invade the village and kill the people he was serving.
On the other side, Luffy was very genuine in his will to help people who were kind to him in that village, and did everything in his power to avoid its destruction, while Usopp ensured that everyone would be protected and everything would be kept secret, in order to have the village keep its usual peace.
- Don Krieg
While Don Krieg’s dreams technically didn’t get squashed, the hopes and will of his crew have been, which means that continuing his journey into the Grand Line is essentially impossible.
The appearance of this giant galleon, coming onto the Baratie like an enormous ghost ship gives us all the information we need: their journey has been brutally stopped, and life has been sucked out of the crew.
While Don Krieg does wish to get back onto his journey and depends on the fear that he gives to his crew members to keep on going, their loyalty has been killed in the process of their utter destruction. Their entry on the Grand Line has been the end of their common dream.
It was revealed in Marineford that Crocodile had faced Whitebeard and completely lost against him, to the point of seeing his dreams and ambitions being completely crushed in the process.
Given what has been discussed so far, it is therefore not a surprise to see that he has decided to settle down in one place with a whole new ambition: destroying the world with an Ancient Weapon.
It is quite striking how he wishes to kill Whitebeard, if he is given any chance to do so, which makes his will to find an Ancient Weapon quite logical. Whitebeard being a force of nature, nothing less may work against him, and it is perfectly understandable that Crocodile would try that option in order to work on the deep bitterness inhabiting him.
- Gecko Moria
Moria truly is the embodiment of depression due to grief.
Not only is the Thriller Bark widely considered a representation of depression (specifically with Brook’s story), but his character arc shows a behaviour that may be attributable to it.
As a young pirate, Moria had his own crew, but attacked the wrong enemy, as he went against Kaido, an Emperor of the Sea, and saw them all get annihilated.
Regretful and bitter, he turned back from Wano country, taking with him the body and sword of Shimotsuki Ryûma.
Beginning there, he would start creating a whole new crew, essentially made of zombies who would not die, thanks to the shadows that he would collect with his Devil Fruit.
It is very striking to see that he warned Luffy about the dangers of the New World, telling him very bluntly that all the crew would get obliterated -which did almost happen, one arc later.
It shows his bitterness and his trauma from his previous loss.
It is also to be noted that he seems to have very close and loyal crew members, in the characters of Absalom, Dr Hogback and Perona, who form the core of his crew, for whom he seems to be ready to do anything.
It shows that he had been full of hope one day, and that his love for the people closest to him stays intact despite his bitterness.
Not unlike the others, he was met with one of the three pillars of the world, in the person of Kaido, the Emperor of the Sea.
As the most recent one, Kaido might be the person who is the most vivid in our memories.
Kaido has arrived in Wano, already shattered by a trial that he failed.
He took the control of Wano, destroying the hope of the people by placing a man who hated the country as Shogun.
What makes him a failed Joy Boy: it is shown, in several instances, that he seems to have thought that he would be the next Joy Boy, but that he has come to the conclusion that he in fact was not.
He gained powerful allies, who revere him in a way that shows the impact that he had on them before losing himself (specifically, he seems to have adopted King, Page One and Ulti at a young age, and Jack seems to have unwavering loyalty to him).
Kaido seems to have come to the conclusion that he was not able to protect what was important to him, to the point that he is drinking himself silly and is insulting himself over his failures, using the present to reflect on the past.
Kaido, as an Emperor of the Sea and a heavy influence on the world, has a lot of power in his grasp, and has made the choice of using artificial Devil Fruits to make his crew more durable, as in reaction to a past crew being wiped out due to his weakness.
What makes them fail
Each of those characters have something in common: an ambition that hit too great a wall.
Kuro met the danger of the Marines, and chose to retract out of the sea and into the lands of a little East Blue island.
Don Krieg entered the Grand Line, and saw his crew be put to death by a single man- Dracule Mihawk, the Warlord of the Sea.
Crocodile was destroyed by Whitebeard- an Emperor of the Sea.
Gecko Moria was met with the utter annihilation of his crew- due to Kaido, another Emperor of the Sea.
Kaido himself, from what we know so far, has probably lost his entire crew- given the pattern that we can see, it would be safe to assume that his first crew fell victim to the World Government. Should it refer to the Rocks Pirates, which would point towards the Garp-Roger alliance, or to his many captures by the Marines, we will probably learn about it very soon.
This summary however makes this obvious: all those people who may have become a Joy Boy have been destroyed by one of the three pillars placed by the World Government to keep balance in the world.