We are in the age of a digital economy. Also, in order for the British Pound to be replaced, a universal currency had to rise high enough to take over for it. America was clearly the largest victor from WWII. Today, the Chinese and the EU are sitting on potentially competitive currencies. In a digital age, you could see the US dollar crash in under a year, and foreign powers could divest themselves of US currency holdings over their laptops instantly.
The Chinese and Japanese and EU still are still holding too much of USA treasury bonds etc to want to cause a US Dollar crash. They would first divest in them first which is what they are doing.