It's not so much about Luffy's will but the circumstances of his birth. But then insinuate that the main character is a child of destiny or otherwise the only one capable of obtaining the treasure and that the world has conspired to make him successful, then the core foundation of the show shakes.
Sprinkle on top of it mystery and intrigue and it's no wonder why it's successful. Add onto that the vibrant cast of characters with creative powers for an engaging show.
The concept of an open sea with a universally appealing treasure ready to be taken by whoever has the will to take it is a stirring concept.
