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.
Not going to completely disagree with you here, but OP’s illustration shows that they were asking about essentially jumping off the red line from the section of the mountain between the pathways from the North and West Blues.
OP simply seems to be disregarding that unlike the terminal at Mariejois, there’s no infrastructure to allow ships to cross over the Red Line. This is a supercontinent we’re talking about here (on a world much bigger than our own). The brevity of the journey through Reverse Mountain gives the illusion of a relatively small mountain, but in reality it’s at least comparable to some of the largest mountains in our world.
Assuming that you didn’t need the rode poneglyphs, a better option would be to steal a marine ship and sail through the end of the calm belt. I’m sure there’s other methods we don’t know of (multiple people who travel the grand line have been able to exit it: Mihawk, Shanks, Dragon) but this is the easiest option ATM.
