That's true but not very meaningful. Imagine the difference if only US citizens would take USD (and only so much of it) instead of it generally being preferred world-wide.

So yes, a government could set up an e-currency and make us use it, but there's no way a government could (intentionally) set up a currency like bitcoin. Truly algorithmic e-currency is fundamentally incompatible with the concept of governing. Anything by a government would allow reversals, currency injections, invisible transactions, etc.