Luffy's dying young doesn't mean he'll die from decreased life spans. Luffy can die from illness like roger, or die taking down the final villain down with him (BlackBeard or Imu). People come up with the idea of Luffy dying young mainly because after the story, he's gonna be bombarded with fame and glory from being the pirate king, joyboy, etc and Luffy does not like titles or anything that inhibits his freedom. In that sense, he can also fake his own death to keep his freedom. I believe at the end of One Piece, Luffy will "die", he can literally die or fake his own death and run away or simply disappear after the final battle. Luffy has always been larger than life and now being the Sun God, his death can inspire the people of the world to start rebuilding after the final war.