America supports a free-trading global regime, bolsters financial stability, and has a giant domestic market. Beijing can play a larger role, but it’s much less of a free-trader, has less influence in international institutions like the International Monetary Fund, and the yuan would uncomfortably substitute for the dollar.
If China were to sell more of its yuan, its currency would strengthen, hurting its exports, which drive its economy. How long would Beijing tolerate that at any major level? A weakened America might also resort to more protectionist practices to create more jobs at home, triggering dangerous trade wars.