People keep referring to the US$ as the World Reserve Currency. This is not the correct terminology (I got this from Adam Smith in his book, Paper Money, 1982). The Reserve Currencies are all those currencies kept in reserve by international banks. These are the US$, UK£, €, Swiss CHF, and Japanese ¥.
Before, the two major currencies were the UK£ and the French ₣, but after WWI, both were weakened, and after WWII, both quickly descended to a fraction of their old value, while the US$, guaranteed to be worth 0.88 gm of 24K gold (approx: the real value was 1/35 Troy oz, but even those who know a normal ounce have no idea what a Troy ounce is, but 1/35 Troy oz is approximately 0.88 gm).
So the US$ became the world's Key Currency, all other currencies had their value defined in US$. So, in the absence of any alternatives, the US$ became the currency of just about all International Trade, since the value of the US$ was set in 24K gold, while other currencies values varied, so a seller did not want to be paid in the buyer's currency which might be devalued, nor did a buyer want a contract to pay in the seller's currency which might be up-valued. So international transactions were (mostly) denominated in US$, making the US$ the Key Currency.
For years, the US has attacked and forced regime change in horrible dictatorships with massive armies that threatened the entire world: Grenada, Panama, Iraq, Yugoslavia, Afghanistan, Libya, Syria, and similar. Now, the US is taking on the weak, impuissant Russia and the PRC, which should be much easier than any of their predecessors, since both are weak, primitive nations without major militaries. At least according to US intelligence (an oxymoron).
So now the US is starting with severe sanctions. Note that, when Saddam said he wanted to be paid in Euros, not US$, the US military saw that he was hanged. And when Gaddafi said he wanted to be paid in Euros, the US had him killed (the US had been trying since President Reagan, but Obama finally got him, confirming the validity of Obama's Nobel Prize as America's Best President EVER).
Russia is no Libya nor Iraq nor Panama, but is now (according to US intelligence) so weak that regime change should be a bowl of borscht.
After all, what does Russia have? One of the top 3 exporters of oil and gas (didn't help Saddam nor Gaddafi); top exporter of corn; leading exporter of metals and fertilizer; & etc., but the world can do without Russian oil or gas or corn. Prices have doubled, and will treble or quadruple (if we're lucky), but we still have to sanction Russia, so we're just going to have to live without heat in winter nor A/C in summer and not enough food.
But another thing: Saudi now accepts payment in c, and Russia accepts payments in ₹, something we haven't seen since Bretton Woods, 1944. And if other currencies become common for International Trade, those currencies will rise in value. And then the world will not have a Key Currency!