In fact, since ancient times, the global currency system is of super-sovereign. Silver standard and gold standard both have precious metals as evaluation. Wealth means silver and gold, not printed money or bank account. At the same time, if the world currency is linked to the sovereignty of a country, then the power would dominate all others to maximize the seigniorage, damaging the whole system.
Not all monetary systems were monolithic or state based. Proof of work has been used by aborigines who fashoined stones into various shapes using hand tools to mint money. We have precursors to modern cryptocurrency PoW from ages past to draw from.
The biggest issue with international reserve currencies is trust. Everyone wants to do business with a nation who can be relied upon to manage their native currency in a friendly, stable and reliable format. This arrangement works over the short term, with everyone being happy. Over time responsible handling of currencies naturally deteriorate. People take for granted basic protections built into the system to prevent catastrophic collapse. They think "things will be different this time" and decide to relax safeguards and deregulate. Lessons of past crisis like the Great Depression are forgotten, opening the door for history repeating itself in a bad way.
Bitcoin being a "trust less" system grants it potential to avoid currency based boom and bust cycles, common throughout history.
Today even metals markets can be prone to high market manipulation. As exhibited by common metals like aluminum having artificial scarcity created in markets through shipping issues deliberately built into supply chain networks. The internet and information ages have opened the door to widespread market manipulation, which were impossible in previous eras. The idea of a gold standard and natural market mechanics guaranteeing some semblance of stability, no longer applies.