I disagree. A flat tax keeps payroll taxes. A flat tax keeps withholding. A flat tax keeps the IRS. A flat tax doesn't stay flat (read history). A flat tax is a VAT. A flat tax means the feds still have a need and the right to know how much you make and how it's made. And a flat tax means you still are at the mercy of the arbitrary, capricious income tax code.
BTW, ALL income is invested tax free under the nrst - not just 401k money. Further, all home mortgage interest will still be paid with untaxed dollars. And, there is no need for personal exemptions under a sales tax - all earnings are always 100% exempt.