The whole "destroy the red-line and make the world 'One Piece'", never really sat right with me
I felt the same way until I properly figured it out. Now I'm dead against it.
If Luffy turns all the world's oceans into a single ocean, that means the Grand Line has to become a just another boring stretch of water. Luffy.. the Pirate King.. destroys the Grand Line.. the home of adventure..
Ah.. but Luffy does it because of the Will of D. and destiny and the "inherited will."
So.. the Pirate King does what he's told? The man with the most amount of freedom in the world.
It also doesn't make sense from the World Government perspective. Why would a world government, the literal embodiment of globalization, want to fight the ancient kingdom to stop them from making it easier to travel around that globe? It's the Grand Line that causes most its problems. It's the boring four blues where the government holds all most power. Why wouldn't they approve of the entire world becoming like the blues? Also, why would the government fear the void century so badly if that's all that the big secret amounts to?
Also, why didn't Roger do it himself? If the One Piece is something that can be achieved so simply, why couldn't Roger just do it himself. He had a few months and he was already one of the strongest men in the world.
I think there's a nugget of gold in amongst the nonsense though. I think Luffy will destroy Mariejois, accidentally destroying Fishman Island, and creating the All Blue, but leave the six other seas more or less intact. Pirates and adventurer's can go through Reverse Mountain while traders and tourists can go through the All Blue to any ocean. Since Luffy doesn't need some contrived reason to destroy the whole Red Line, or Reverse Mountain, there's no reason to believe it has anything to do with the ancient kingdom or Will of D. Luffy destroys Mariejois because HE wants to. Noah's exists because fishmen in ancient times wanted to live on the surface same as they do now.
But alas.. it seems the "Inherited Will" theory, as it is called will, remain popular.