I am not aware of any real long term metrics to track reserve currency usage worldwide. One of the many ways the dollar system is reinforced is by the old idea of the petrodollar. Some would say this has been in place since the 1970s. It's not the entire picture of what makes the dollar the reserve currency, maybe the U.S. Navy can ultimately take credit for it along with the historically liquid financial system behind it.

With that being said, much of this system relies on the Treasury market. Saudi investment seems to move based on which political party is in power. Russia has a complicated relationship to the petrodollar as well and has grown more complicated since Crimea in 2014...

Part of the staying power of this system is arguably technical because of SWIFT. The double edged sword of using sanctions for geopolitical goals has driven other means of settling international payments even for things like oil. It's less ideal than the system used to be but in the oil producing countries, it's better than being at the mercy of the U.S.

Even Japan, which acts as a sort of off shored Federal Reserve given their historical bond holdings has recently cut back and said they plan to cut back more.

China also has complicated the picture. The trade war changed many fundamental inflows and outflows and as time goes on more countries will find ways to trade without relying on the dollar.

This can be patched over by something like quantitative easing where the Fed steps in to provide liquidity that global trade used to provide, but there is a cost to that as well since they are kind of stuck in a trap of inflation vs job loss and recession. Assuming inflation continues.

Don't get me wrong though, the US dollar is still the reserve currency and will be for some time. It's just that the mechanics behind it can and do change and is something that I feel is not widely understood.

How about that Bancor though?