You can't hand-wave away where this money comes from.
“This is the United States government. First of all, you never have to default because you print the money, I hate to tell you, OK?”
You could break it. It's not immune to the hyperinflation that's hit other currencies.
Historically, the only other proposed option was the EU and much as been made about unsustainable financial policy there (many allege these issues are overblown to help discredit the notion of a strong Euro, but that's neither here nor there). But now we're starting to talk about creating to-order currencies with arbitrary properties, guaranteed supply limits and curiously popular ownership policies.
While governments will never give up the power to mint their own currency, it's not unimaginable that in 10 years some some countries will seriously consider participating in a digital reserve.