Let's say that when An Lushan is on his way to visit the Xuanzong Emperor in Chang'an in 743, he has an unfortunate horse-related accident and dies. He passes into history as a regional commander of little importance.
Without the An Shi Rebellion, what happens to the Tang? Can they hold on to their Central Asian territories for longer and maintain contact with the Caliphate? Without the central government being disastrously weakened and millions of people dying, how much longer can the Tang last?