If you follow the rules strictly, each game ends with the last turn being completed. However, you are free to end it earlier. In your case, a major Japanese victory was obvious...
Let us say that on the last turn of the game the Japanese conduct an attack during their combat phase. Even if they have carriers to return to, are the airplanes lost because they do not have the necessary turn remaining to land back on their carriers? The answer would be important for determining victory points.