The prophecy of Sharley says that the man with the Strawhat will destroy the Fishman Island. Fans always speculated that Luffy is going to destroy the Fishman island by destroying the red line, but I don’t think so.
Ten years before the current storyline, Queen Otohime tried her best to gain support for the migration of Fish-Man Island to the surface world, and made many speeches in hopes of getting petition. Despite her efforts and popularity, she gained very little support, as the relationship between the species were strained.
Later, the Celestial Dragon Mjosgard came to the island to retrieve the fish-men slaves that Tiger freed, but his ship was attacked by Sea Kings, leaving him the sole survivor. The former slaves tried to kill him, but Otohime saved him, and accompanied him back to the surface, retrieving a support from the Nobles to help the island’s immigration. This led to many support from the islanders once again.
Joyboy attempted to raise Noah with Poseidon’s aid, but he broke his promise to Fish-Man Island. However, Fish-Man Island promised that they would keep Noah safe until a certain day came. Fish-Man are waiting for the day they will come to the surface again.
The goal of Fish-Man is to come out to the surface, to be free, but the problem with Fish-Man is that they have fear of not being accepted, that they will be discriminated, hated, hunted, enslaved and killed. The mutual hate between them and the humans comes from the fear of the unknown.
You would think that Luffy, hearing the story of Fisher Tiger and Queen Otohime, hearing about the dream of Fish-Man to live on the surface, hearing about their suffering and suffering of the man who saved his life(Jinbei), that he would say something and do something. But, Oda purposely made Luffy miss the whole story Jinbei was telling by making Luffy fall asleep.
In the future, Luffy will learn the truth about Fishman island, that they are there, not because they choose to be there, but because Fishman-island is a mental prison of Fish-Men and Fish-Men are too afraid of leaving their own prison, and Luffy will do the only thing he can… He will destroy their prison, make them realize that they are free from the humans and bring them to the surface using Noah and Poseidon, thus fulfilling Joyboy’s promise and Sharley’s prophecy.
This will parallel the story of Nami who was once mentally imprisoned, by a Fish-man Arlong, in the cartography room and Luffy did the only thing he could.. He destroyed the room that was prison for Nami, and he let her know that she is free from a Fish-man.
Theory by BuggyDClown.