4) Conclusion: Luffy’s pull
Luffy is established to have a very scary power: his pull, meaning that he’s meant to have many allies who are led to truly believe in him.
This is something, according to the flashbacks we have been able to see, that was a common denominator with the former Joy Boy contenders: people believed in them and followed their pull.
This would mean the following: what makes people follow Luffy is the power of his dreams and of his conviction to be able to go through with them.
In other words: his dreams revitalise those of the people who surround him.
Like Luffy almost saw happen, we can safely assume that our failed Joy Boys, and especially Crocodile, Gecko Moria and Kaido, have let their regret and guilt consume them entirely, not allowing them to see forward, and only making them look at the young pirates in scorn and disgust.
Given that their dreams seem to be lost because of the three pillars put to place by the World Government, it could be considered that they are being forced into being puppets of it, by letting their grief grow into bitterness towards the next generations.
However, this is where they failed and Luffy will succeed: while they seem to have turned to external ways to deal with their loss (looking for a powerful weapon, creating immortal zombies, using Zoan devil fruits to make their crew more powerful and durable), Luffy has turned inward and has perfectly understood that the way to protect his crew is to become more powerful, and to have them become more powerful. He has understood, and the crew has understood, that the only way to make his dream come true is to believe in him and in each other, and to find ways to grow in power as they advance.
When a “Joy Boy” loses their dreams, their goal seems to become squashing those of the people coming after them, like a vengeful spirit unwilling to accept that someone would succeed where they failed. Like ghosts, the former generations are haunting the seas, and refusing to give room for the living to try where they failed.