> The value of the currency comes from the demand for it by the government to pay taxes, not the value of the metal in the coin.

That's the fantasy, not the reality.

> The metal in the coin makes it expensive to counterfeit said coin, with punishment by death doing the rest of the disincentive.

What actually happens is the government declares a dollar value for the coin, and then alloys the gold with cheaper metals. This results in inflation. The usual content of counterfeit gold coins is less than the gold in the government issue, which is where the death threat comes in.

Only one counterfeiter ever put more gold in the coin than the government, that was Baraha. The government got mad at him because a Baraha sovereign was worth more than the government issue.

> he dictates their value by fiat

Governments always try that, and it never works. Governments are always alloying the precious metal with cheaper metal, but nobody is fooled, and the result is inflation.

Why do you think the government no longer issues gold coins? why there's no silver in a dime anymore?

Inflation.