Just for reference, Amendment XVI reads: "The Congress shall have the power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration."
You are correct, but it wouldn't be just some future Democrat that would start applying both a NRST and the 16th Amendment. There are plenty of tax and spend RINO's who would do the same thing.
Socialism: It's not just for Democrats anymore.
Any passage of a national sales tax could be conditioned on the ratification of a constitutional amendment repealing the Sixteenth Amendment and rendering federal income taxes unconstitutional. The amendment could incorporate a phase-out provision so that we could have a transitional period during which we phase out the income tax and phase in the sales tax.