Anyone trying to design an event to bring Xi Jinping’s China back to Earth couldn’t have engineered something much more elegant than the turmoil in China’s financial markets and the resulting global aftershocks. The upheaval is traumatic for China’s leaders but not life-threatening to China’s system. Yet the jolt may have been just large enough to change the country’s underlying bargain between ruler and ruled—and by doing so, to temper Beijing’s current tendency toward arrogance, rigidity, belligerence and diplomatic hectoring.