Very good question. First, I'd like to make it clear that I didn't mention bitcoin in my original message. But I don't think you can compare a fiat reserve currency to a cryptocurrency. They operate in different ways. For example, the US dollar is controlled by the Fed. A central bank is an economic institution that serves political purposes. Human beings are behind the decisions that affect the value, liquidity, etc. of the US dollar. Bitcoin doesn't have those problems. It does have rules in place, like the recent halving, but there are no surprise rate hikes or cuts, no sudden unlimited QE that has to happen in order to prop up markets. When foreign nations decide to use something as a reserve currency, they need more than just someone's word that it will hold value, because they know the human beings who are controlling it probably aren't completely honest, and they don't really know these people are competent either. Human decisions are not predictable, and a currency, especially a global one, needs to have predictability. Central banks have essentially turned currencies into commodities. If you remove the question of human decision making so that the whole world could know ahead of time how the US dollar would behave moving into the future, nobody would care very much if it is backed by anything.

Your question brings up some other valid questions of course. What backs gold? What backs all those stablecoins that claim to be backed by the US dollar? If the dollar isn't technically backed by anything, how could it back a stablecoin? Perhaps the dollar is partially backed by the demand created by those stablecoins. Will bitcoin ever be a reserve currency? If the US goes through what the Weimar Republic went through, what would the world trade in? If you think "it can't happen here", remember that it couldn't happen there either, until it did.

I think those are the big questions as this topic relates to bitcoin. A reserve currency needs global trust, but more importantly the world needs to trust what is behind a reserve currency. It isn't the dollar that people don't trust; it is the Fed. Bitcoin isn't inherently more trustworthy. The reason it seems more trustworthy is because it doesn't have a central bank.