Well, from what I understand of the theory, the boils down to this: The ancient kingdom planned to destroy either the entire Red Line or just Reverse Mountain/where Mariejois is currently. This was a plan to make all the oceans in the world into a single ocean.. hence making the world into "One Piece," AKA the All Blue.

The proto-World Government disapproved of this.. for some reason.. and wiped them out to stop it from happening. Luffy's going to find all this out and carry out the plan.

Now.. from the way it's presented, the Grand Line is going to become just another stretch of ocean, similar to the four blues.

Why would Luffy destroy the Grand Line? Why would he want to destroy the home of adventure and the place dreams come true? It doesn't make sense and it goes against one of core principles of One Piece. I refuse to accept that the great action of the Pirate King is to destroy adventure.

When I've brought this up before, people have said that it's because of the "will of D." and because he has to in order to make the All Blue or whatever.. But that goes against the very idea of the Pirate King, that he's got the most freedom in the world. Luffy doesn't have to do anything he doesn't want to, and he would not want to destroy the Grand Line.

So from the start, it's already going against two major principles of One Piece, adventure and freedom.

It doesn't make sense for the World Government, the literal embodiment of globalization, to want to restrict movement around the world. In fact, they should welcome it, seeing how it's the four blues where they hold the most power, and the dangerous Grand Line where their authority wavers. They'd love to make the Grand Line just another stretch of water.

Also, it doesn't make sense as to why the government would want to restrict knowledge about the Void Century. Some ancient kingdom wanted to try their hand at a bit of terraforming? That's the incredibly dark secret that threatens the government on such a fundamental level that the slaughter and entire island?

Finally, Roger. If the "One Piece" is as simple as to destroy something then everything is changed forever, then why didn't he do it himself? He had months before he handed himself in, and he could read poneglyphs, and he was on friendly-ish terms with the man who could create earthquakes. Why couldn't he have sorted it out himself rather than dick around with the pirate age? That implies, to me, that the secrets of the void century is not a quick fix that permanently changes everything, but more likely a society or way of life that must be rebuilt and maintained.

Like I said previously, I think there is a good theory in there, but it's been artificially inflated to be all encompassing.

Luffy destroys Mariejois because HE wants to. That destroys the Red Line under it, destroying Fishman Island. In the place that's left the All Blue is created, leaving the other six seas intact. Now, tourists and traders can go through the All Blue to reach any other ocean, and pirates and adventurers can use the traditional entrance of Reverse Mountain to enter the Grand Line. Noah was built because in ancient times the Fishmen wanted to move to the surface, same reason they do in the modern day.

Since Luffy doesn't need some artificial reason to destroy Reverse Mountain or the rest of the Red Line, its destruction, the creation of the All Blue and the destruction of Fishman Island doesn't require it to be a plan from the void century, nor does it need to have anything to do with the Will of D. or whatever.

I plan on making a video about it.