If someone saved money in a conventional savings account or Roth IRA, they've already paid income taxes on that money, in the expectation that they won't have to pay federal taxes on it again. Replacing the income tax with a sales tax would mean that people who buy things with borrowed money before the switch and repay the debt after would not pay any taxes, while those who save money before the switch and buy things after would get hit with both taxes.
What FreedomCalls means is that he's concerned that it isn't fair that someone who has done the right thing to save for retirement would have to pay extra just because of changing to an NRST.
It's a common concern. But you're already being double taxed on your spending today. Indeed, every single thing you buy has 20-30% invisible tax in it. In today's world, you're taxed when you earn it and taxed when you spend it on ANYTHING - even the necessities of life.
Under the nrst, only certain spending is taxed - income is never taxed. Under the nrst, there is no gift tax. Under the nrst, there is no death tax. Under the nrst, seniors do NOT have to pay anything more than they were going to pay anyway - and seniors do not have gift or estate tax.