The worldbuilding of One Piece, as you probably know if you caught even one episode of the thing, is one that is incredibly hostile to its inhabitants. A lot of pirates are bitches, obviously, it's all in the title. A lot of the marines, supposed to stop them, are also bitches. A lot of the royalties are bitches. On top of all these people stand the Celestial Dragons, people with absolute power over everyone, and wouldn't you guess it! They're even bigger bitches than everyone else! If there is an organisation upholding order in One Piece, then it's corrupt and built to exploit the ones beneath it. Some individuals are exceptions, but none of the actual organisations are.
You would think then, that the world of One Piece is one that is hostile to the weak and the vulnerables. And, well, yes, it is. If you're not strong enough to stand for yourself, you're majorly boned. But that would also be a pretty reductive statement, because it would imply that strength could in fact save you from the misery of the world. And it cannot.
None of Big Mama's family are happy. Sanji's sister is not happy. The celestial dragon who went to fishman island is not happy. Because even if they have power, even if they're at the top of their food chain- the second they act differently from what is expected of them, they get punished. Bullied, harassed, tortured- hell, this one Celestial Dragon who changed his mind, a guy who legally can do whatever he wants all the time forever, was tortured and sentenced to death for daring to suggest that maybe! Everyone should chill out on the slavery.
The world of One Piece is not simply hostile to the weak. It's hostile to the free. Any act of individuality, of deviation from the norm, is to be punished. These systems are rotten from the core. There is no changing them from the inside. They're built to make you into a gear or crush you in its cogs.
And that's why Luffy is the protagonist here, the one to kickstart so many changes in this world. Because Luffy is the embodiment of freedom. He wants freedom more than anything else in the world. He does whatever he wants regardless of consequences (and lord knows sometimes the consequences are bad! Punching a Celestial Dragon was a very bad move for everyone involved!) Luffy isn't a hero, isn't some good guy trying to save everyone. But he is Free. And in a world like this, this is the biggest threat possible.