The US government is ultimately beholden to US citizens, who don't want to get hosed by inflation. People who can't vote in America do actually have a legitimate reason to be concerned: if the US ever can't maintain its debt, it will likely become in the voter's interests to devalue the currency to nothing. That's basically the "default risk" on Treasury bonds.

Approximately 300m people vote for, pay taxes to and support the creators of US dollars, the US government. Approximately 8 billion people recognise the US dollar as a global unit of value exchange which has a known local currency value.

A new cryptocurrency is just a database with some private keys shared remotely.