If the one piece is at the end of the grand line, what stopping someone from just taking a shortcut to it like this? Yes, the drawing is shitty, but it should at least get the point across.
The short version is “that’s not how Reverse Mountain works.”
The long version is that Reverse Mountain pulls in from the four Blue Seas and spits you out in Paradise (the first half of the Grand Line). Even if you could sail backwards through Reverse Mountain (you can’t, the water flow is insanely fast), you’d just end up back in one of the four Blue Seas.
Also, the One Piece isn’t “at the end of the Grand Line”, it’s on Laugh Tale, an island located somewhere in the New World (the second half of the Grand Line) that can only be located using the four Road Poneglyphs. Even if you did end up taking a shortcut to the New World, you still couldn’t find Laugh Tale without the Poneglyphs.
