even currency doesn't have a value of its own
No, currency has intrinsic value because the government says it does. I know that sounds stupid, but bear with me - a dollar will always be worth something in the United States, even if every store decided they would only accept gold, bitcoin or wolverine teeth, because taxes are payable in dollars alone. That power to tax is what gives fiat currency its value.
If you choose to live your life doing all your transactions in wolverine teeth, you will still need to trade those for dollars at some point in order to pay your taxes.
Note that this doesn't imply any value for the dollar outside the US. All the people in third world countries who conduct gray or black market transactions in US dollars do so because of the value created by the US government taxing its citizens in dollars, and the fact its citizens travel and do business around the world. If everyone thought like Trump and the US became isolationist, the US dollar would become much less attractive for those outside the US even if it maintained its value for US citizens.