We don't have a single currency in the world today, and not even a single reserve currency (where presumably the network effect of liquidity could reign supreme, unfettered by legal tender laws, etc.). I doubt in the history of civilization we ever really have, except perhaps in some isolated simple economies. If the dollar fails, then we can turn to sterling, or bitcoin, or renminbi, or gold. The world has a way of recognizing fragile over-optimization and avoiding it, though sometimes this process takes the form of going to far and having a catastrophic failure first.

One single coin to rule them all is a catastrophe waiting to happen. I doubt that happens though. People recognize the risk and avoid it, which is likely why something like LTC has any value at all (though LTC isn't really the best for this and will likely be overtaken). It isn't for features or even marketing.

The Bretton Woods system was the first example of a fully negotiated monetary order intended to govern monetary relations among independent nation-states. The chief features of the Bretton Woods system were an obligation for each country to adopt a monetary policy that maintained the exchange rate by tying its currency to gold and the ability of the IMF to bridge temporary imbalances of payments.
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What emerged was the "pegged rate" currency regime. Members were required to establish a parity of their national currencies in terms of the reserve currency (a "peg") and to maintain exchange rates within plus or minus 1% of parity (a "band") by intervening in their foreign exchange markets (that is, buying or selling foreign money).

In theory, the reserve currency would be the bancor (a World Currency Unit that was never implemented), suggested by John Maynard Keynes; however, the United States objected and their request was granted, making the "reserve currency" the U.S. dollar. This meant that other countries would peg their currencies to the U.S. dollar, and—once convertibility was restored—would buy and sell U.S. dollars to keep market exchange rates within plus or minus 1% of parity. Thus, the U.S. dollar took over the role that gold had played under the gold standard in the international financial system.[20]

Meanwhile, to bolster faith in the dollar, the U.S. agreed separately to link the dollar to gold at the rate of $35 per ounce of gold. At this rate, foreign governments and central banks were able to exchange dollars for gold. Bretton Woods established a system of payments based on the dollar, in which all currencies were defined in relation to the dollar, itself convertible into gold, and above all, "as good as gold". The U.S. currency was now effectively the world currency, the standard to which every other currency was pegged. As the world's key currency, most international transactions were denominated in US dollars.