So, how does that differ from what they can and have done with the current tax code? Yeah, there isn't a national sales tax, but if there were, all those not currently paying taxes in the bottom 47% (whatever the percentage is) will have to pay something and 9% income tax payers will have more disposable income than they do now to cover the additional sales tax.
9-9-9? Yes! Yes! Yes!
If your argument is that all taxes can rise over time and 9-9-9 doesn’t impose a constitutional limit, then 9-9-9 will rise over time. History has shown that.