It's a good theory, but I still think that the One Piece isn't something palpable. It could represent the freedom, the truth (including about the void century), the power, or probably all of those together.
What kind of treasure would be greater than knowledge, strength and freedom ?
During the whole story, the knowledge of the SH is growing up, Nami is about to map the world, Robin to learn everything about the void century, Chopper to learn how to treat any disease, Sanji to cook anything, Luffy how to protect his crew anytime… They are about to acquire the biggest knowledge of the world... Geography, history, gastronomy, health, fighting, music, ballistic, engineering...
On the same time, they all deserve to be strong, stronger than any ennemy, in order to protect themsleves from any threat.
And if they are able to reach the last island of Grand Line without being stopped, captured or defeated by anything, then that would mean they are quite unstoppable, and free.