Think of the trust that people have towards fiat money. So, we had USD backed by gold 1:1, and they claimed that each Dollar can be exchanged to gold at request.
There was a situation where France wanted to check if USA really owns enough gold, so they asked for the US government to exchange all their fiat money to gold, but the US government refused. It broke the deal and took US off the gold standard. They basically told the world that they won't honor the deals they make, but people still kept USD. Everybody should've dumped at that point because that's what would happen to a coin like BTC if Satoshi came and said that he's going to change the protocol to whatever he likes, like that there will now be 30m coins instead of 20.

The current value of USD is greatly based on trust. Fake trust, because people should not trust the government that does not honor deals. Bitcoin is much better in this because the rules are simple and known to all parties thanks to its transparent protocol. If countries can hold fiat money belonging to other countries, they can hold bitcoin. For instance, the US holds EUR in its reserves. Why would you need billions in a foreign currency, unless its a part of some mutual exchange deal? We will hold your money, you'll hold our. Why wouldn't they convert at least 1% of that into bitcoin?