Monkey D. Luffy was not meant to grow old.
Luffy knew it and his nakama knew it, even when they tried not to see it. People like Luffy – people who burned so bright, so that one had to avert their eyes or be blinded forever- they weren't meant to grow old. They were born martyrs. They were to carry their destinies out, reach one pinnacle of extreme triumph, and die at the height of their power.
Just like the first Pirate King, the second would also have to fall. No one wants to see a hero after his peak of perfection.
Monkey D. Luffy knew this, even when he did not always accept it. It lingering the back of his mind – sometimes like a lurking shadow threatening to consume him, sometimes like the single ray of hope that guided him. It existed in his silence when his Nakama spoke whimsically of life after Raftel. Oh he would find the One Piece and he would see his nakama's dreams become real life but he knew that his life would soon be his legacy.
His future was a canvas for which he had no brush. Others ran their fingers over the pristine white parchment leaving oily stains which Luffy did not fight. Settling down, he would never. His gaze was on his body, his present. He ran lines of red and black and steam onto his skin with his own fingers. He swirled the colors together into a living masterpiece. His hand was directed by fate and the result was inhumanly perfect - unsurpassable.
When the masterpiece was complete, when fate had finished its course – used his body like a glove to cover a master's hand – when his purpose was served, Luffy died. It was a loud and vicious death like that of an exploding star. Those who had gazed upon him in wonder for so long were suddenly forced to look around the world around them and shudder. It was a darker place, less alive, for Luffy had lived harder and more desperately than anyone. Those who were lucky enough to have existed around him and had been exposed to his vitality were at a sudden loss without him to fuel them. They might live forever, as they had never lived at all. But they would have to remember and live on for him.
Monkey D. Luffy lived a life that was hard and demanding and thrilling and primal. He clung to his life as a drowning man claws at a piece of driftwood trying to stay afloat. He would die young and violent, and for that he would live more than anyone.
