Indeed an interesting topic. The demand for dollars largely stem from the ability to use them. When doing international trade you want to be paid in a currency you know is fairly stable and exchangeable to other goods and services with relative ease.

When sovereigns in developing countries issue bonds they tend to need to do it in USD to get any takers; else the risk of their inflating the value of their currency away is just to great.

As US dominance decreases it's currency will face competition from e.g. the yuan. Ray Dalio writes about this in his "The Changing World Order".