I agree with you for the most part, but answer me this:
What prevents them from instituting both systems of taxation now?
Nothing in the tax code, the Constitution, or the law will do that.
The only thing that constrains them now is fear of losing their jobs and fear of losing their lives if they push us too far.
Correct and that is the reason they have not instituted both. If they do institute the National Sales Tax, then it would only be prudent to remove the option for income tax at the time of its enactment, since that is when we would have the most leverage to do so. It would be the quid pro quo, so to speak.