@Claudio:
Roger also "failed". Blackbeard would not be a secret World King. If it wasn't for Roger, maybe Rocks would have been World King. No need for One Piece.
All that started long before Luffy was born too. So what? What does Blackbeard being 40 have to do with Job Boy leaving the treasure 800 years ago? That's right. Teach is not meant to do what Roger couldn't but what Rocks couldn't. What happens when Blackbeard does what Rocks couldn't before Luffy does what Roger couldn't?
Luffy is also just a pirate. So what happens if Blackbeard simply beats Luffy to the punch?
Okay, just follow me for a minute.
There is something deeply wrong with the One Piece world. Whatever it is has been happening for 800 years or began 800 years ago. Whatever this thing is, it was so significant that the World Government wiped out 100 years of history (more on that, but I think this is also bigger than erasing a few history books). Joy Boy lived during that time period and seemingly failed. One Piece was left in the most remote, difficult to reach location in the world. Joy Boy's allies, the Kozuki Clan, created Poneglyphs, a secret language, preserving the truth and guiding the way to the final island where the history of the world is hidden. These stones are protected by ancient civilizations, all of whom tell variations on the same story - a day when a messianic hero will set the world right (The Minks - the light of the new dawn; The Fishmen - the hero and the Mermaid Princess who will guide them on the road towards the sun, etc.). They have guarded this secret for 800 years. Many, like the scholars of Ohara, have died in effort to protect the truth. Whitebeard, who knew the truth of the world and the D's, remarked that "When the treasure is found, the world will be turned upside down. The discovery of One Piece is the key, the trigger; It is the impetus for whatever is going to happen to fix the world.
Roger didn't fail. He didn't try. He arrived too early and his days were numbered. He had a terminal illness and the pieces were not yet in place. Shirahoshi wasn't born yet. Shyarly told him the number of years it would be and Roger, knowing he would not be the one to confront the world and reveal the truth, decided to disband his crew. He went and either met or reconnected with Rouge, had a son, whom he believed would be the successor to his will and re-discover One Piece (Newgate also believed Ace would be the Pirate King). Oden returned to Wano and advocated the opening of the borders for the promised day that would come 20 years in the future. Roger then turned himself over to the Government, taking advantage of his public execution, and crafted his dying message to inspire a successor, to follow in his footsteps and re-discover One Piece. As Rayleigh said, Roger turned his dying light into a roaring fire. He intentionally created the Great Pirate Era. Why? Newgate, Rayleigh, Oden, Crocus - many people in the world know the truth. If he believed Ace to be the future Pirate King, why did he not leave the route to Laugh Tale? You have to ask why he left it to fate to decide the future Pirate King and successor to his will. The only answer can be that both the journey and the destination are important. It is not only vital that someone discovers One Piece, but that their voyage follows along the correct path to reach the final island. What is Luffy's greatest power? His ability to make friends / allies of anyone he meets - even those who were once his enemies. See Crocodile and Buggy at the Battle of Marineford as an example. The person who faces the world with the truth, with One Piece, will not do so alone. They must have friends around the world, who can only be made through a long and difficult voyage. They can only attain the path to One Piece by courting the favor of those ancient and disparate peoples who guard the Road Poneglyphs. The path has been preserved this way for centuries. It was designed to be this way. Only a certain type of person can reach the final island. Only two have done so from the time the void century began - Joy Boy and Roger. Someone will rise up to claim that mantle soon. The promised day approaches. Newgate, as he died, looked down at Teach with pity and said "You're not the one Roger is waiting for." It won't be Teach to reach Laugh Tale and claim One Piece.
Rocks dreamed of becoming the World King. Roger faced him at a time before he discovered One Piece. He did not know about the great force that must be confronted. When he died, he was not inspiring a successor to face Rocks, the enemy he had already defeated. He was thinking about the enemy he did not have the time to face.
All of these people throughout history have been preserving One Piece such that the right person may someday be able to bring the light of the new dawn. If it were possible to defeat the darkness which already inhabits the One Piece world without the treasure, these efforts would be nullified.
