Why do you think the FairTax would be harmful to Roth IRA account holders?
The whole point of a Roth IRA is that unlike tax deferred With tax deferred investments such as a 401K the government lets you invest a larger amount of money by not taxing that income first, but still taxes that income and all capital gains as they are withdrawn from the account.
With a Roth IRA you do not get to invest pre-tax dollars. You invest post tax income, but the government promises to never tax that money or any gains from investments made with that money every again.
If we shift to a sales tax under the Fair tax the effect on earnings from the 401k is minor at most, because the income will no longer be taxed as it is withdrawn, and you will instead pay taxes when you consume things during your retirement.
With the Roth IRA, all those earnings that the government promised to never tax as income now get taxed through an extremely high consumption tax which completely voids all benefits of putting the money in a Roth IRA. Those people put that money in a special retirement account that they were not allowed to withdraw it from except under specific hardship conditions until their retirement, but the Fair Tax removes all benefits for doing so, and those benefits would be huge under our current tax system.
I've chosen to invest a considerable portion of my income in my Roth Ira that I could have otherwise invested in my 401K and not have had to pay any taxes on that money. I know a number of people that converted other retirement investments to a Roth IRA when they first were offered because they were such a good investment and people didn't think Congress would allow people to keep investing in them for long. Converting them however meant paying income taxes on those investments that had been pre-tax investments.
The second way that the Fair Tax is harmful to Roth IRAs and all savings for that matter, is that it is going to create a huge price inflation spike when it is implemented. The 30% sales tax will be added on to the price of items. This will be offset by greater take home pay, so you will pay more for items, but you will also have more disposable income. The effect on future income is minimal. The effect on your savings from before the Fair Tax goes into effect is that your savings will now buy you about 20% less goods.
Fair tax proponents love to talk about how the increase in take home pay balances out the fact that you have to pay more for things when you buy them due to the sales tax, but they always seem to ignore the fact that no one is putting money into your savings to compensate. You already paid income taxes on those earnings, and the government sure isn't going to give those taxes back to you.
Some people will try and tell you that the price of goods will drop so that the price including the tax will match what you are paying now. That's complete B.S. The Fair Tax is revenue neutral when compared to the current tax system. The sales tax needs to make up for both personal and business income taxes that it replaces. The only difference is the difference in complying with the sales tax versus complying with the current income tax. Those cost saving also only effect domestic products. Imported goods aren't going to see those cost savings, so they are just going to jump in price by the amount of the sales tax.