> even the dollar as reserve is a fickle thing that could change if the rest of us choose to.
No, you have this all backwards. Let's take a hypothetical country -- Fredonia with it's Fredonian Franc. The government of Fredonia decides "I want to be a global reserve currency!". How would they go about accomplishing that?
First, let's define what it means to be a global reserve currency. What it means is that other reserve banks use your currency as their reserve. That means other reserve banks need to accumulate Fredonian Francs, and in large amounts.
Of course banks don't warehouse currencies, they would be holding Fredonian Government Bonds (FGB), which must be bought with Fredonian Francs. So how would they get their hands on large numbers of FGBs? By running trade surpluses against Fredonia.
In other words, to be a reserve currency means you have to allow the rest of the world to run large trade surpluses against you, which means you have to allow your currency to be permanently overvalued relative to the rest of the world -- relative to what it would be valued if there was no global investment demand for Fredonian Francs.
Now, all of a sudden, it's not looking like such a great proposition for the Fredonians, as this means that their own production is disadvantaged in global markets more or less permanently and their financial system has to be huge and swollen. It distorts the Fredonian economy and hurts workers. That would require a very resilient economy and a population willing to tolerate the employment hit to running persistent trade deficits vis-a-vis the rest of the world.
Next, Fredonia would need to develop deep and liquid capital markets, so that the rest of the world would be confident in storing their wealth there. That means a long tradition of rule of law, large turnover at low prices, and huge bond markets. Big enough to absorb the surplus of the entire world. It also means that your interest rates are going to be permanently lower than they would be if there was no excess global demand for FGBs. This means your economy is going to be subject to asset bubbles and financialization -- we're talking huge bond markets that risk dominating the country.
So the economy of Fredonia has to be big, be investor friendly, have strong property protections and rule of law, a grotesquely bloated and powerful bond market, and be willing to run permanent trade deficits vis-a-vis the rest of the world.
Not many nations qualify on all of these accounts, right? China is big enough, but no rule of law, and since their entire economy is based on running trade surpluses instead of trade deficits, they are suited to be the accumulator of global reserves rather than the issuer of global reserves. Saudi Arabia? Nope. The EU? Nope. Simply no one else is both able and willing to pay the price to be a global reserve currency, and that's why it's going to remain the USD. Not only is this not a "fickle" thing, the rest of the world should be grateful that the US is willing to sacrifice the interests of its own workers and production base in order to make sure the rest of the world has a steady supply of USG bonds.
Rather, the real question is how long will the US be willing to bear this burden? When the US was 40% of global GDP, and global trade was tiny relative to global GDP, it didn't seem like a very big burden. But today, when the US economy doesn't dominate the rest of the world and trade is huge, the price of being a global reserve currency is devastating. It means de-industrialization, collapsing cities in the rust belt, rising death rates and a shrinking US middle class, as well as rising Federal deficits.
I can very easily see a future in which the US starts imposing more and more capital controls that prevent foreigners from purchasing dollar assets similar to protections that other nations have, at which point guess what -- there will be no global reserve currency. The free ride, for the rest of the world's producers, will be over.
Trade agreements will need to go back to being settled either bilaterally, or with some kind of specie (gold? silver? bitcoin?). This would be a great thing in some respects, because it would force each nation to not overproduce, but it would cause a huge contraction in international trade, corresponding to a contraction in global GDP and well being, and especially to those economies that rely on running persistent trade surpluses in order to employ their workers -- nations such as Germany, China, Japan will take big hits should they lose the ability to persistently run surpluses.