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

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.