Its a complicated topic as I stated. When the USA went off the gold standard in the 1970's....The fiat dollar was backed the major exporting countries/blocs of the time....Japan, Germany and OPEC. Other smaller economy countries had no choice but to keep dollar as reserve or else they could not buy important goods like oil, cars, electronics, machinery etc... from the major exporting countries. With the collapse of Soviet Union....the newly opened countries stayed in the existing dollar order (including China) since their no other option and the system worked.
China can easily get the Yuan backing from OPEC+Russia in time. Germany now backs the Euro. Japan will diminish as its in demographic decline as well the importance of its backing of the dollar. The USA produces nothing....it owns. It not hard for the dollar to lose its value to smaller economies.
The USA financial system is also large, open and sophisticated enough to perform this reserve currency duty. China will have to have similarly capable financial sector to compete with the dollar.
Lets look at Pakistan today.....we need dollars mostly to pay back our dollar denominated debt and buy oil and gas from OPEC. In reality we can get everything we physically need from the GCC (oil and gas) and China (defense products and any consumer item).
Times have change the dollar is more vulnerable then you realize.