when
Bitcoin is the global reserve currency.

Last part is very beautiful
I doubt it's even mechanically possible. Barely anyone owns shitcoins and out of the people that do, distribution is terrible with people like The Winklevoss and Tim Draper owning them all. In other words, Bitcoin is an even worse mirror image of the failing fiat system and it's lopsided distribution. Bitcoin is fiat^10 in that regard of problematic distribution. Do you think all the buying power in the world is just magically going to go to bitcoin and the other 99.9% of the planet will all happily go off into the night and die? LOL.
No, I'll tell you what happens. Money is a substitute for law of the jungle, and when money no longer functions for the public at large (due to the evil cult of Judaism's usury scams, media disinformation, banking monopolies, etc) the general public then goes back to the law of the jungle and exterminates the people who made it happen - then whoever else stands in their way of survival afterwards. There is no type of plausible scenario where everyone on earth just dies of hunger in the streets except shitcoin holders.
Like it or not, for any type of future to exist that doesn't involve a full-blown Mad Max scenario where everyone just dies in a 200 year dark ages, it will probably require massive wealth redistribution. If that is the purpose of bitcoin, it failed miserably since it's designed to centralize. There was no reason for it to even exist the day the first ASIC was created. It's for all intents and purposes the same thing as a central banking monopoly in current form. You can at least go dig up some gold, silver, or copper with your bare hands out of the ground or mouth of a river. Nobody can make ASICs and buying them is just a scam.
With digital shitcoins you have the problem of BOTH the citizens AND the governments not owning enough of it to make it work. There is enough gold, silver, and copper floating around though that's held by both govt and citizens. Keeping in mind copper is already in much of the coinage. So, yes, a tri-metal system would really be the only way. Govts with not enough metals in the vaults just call the stuff in the ground "deep storage" and nationalize rights of all of it and the mining stock holders get zero. Then they revalue metals to the moon and this fixes the unsustainable international trade problems + allows govts to run their breadlines.