That requires a whole bunch of trust in Congress and I don't think they deserve that much trust.
I think I'd rather keep the absolute abomination that is the income tax rather than give them the power to impose a sales tax without repealing the 16th amendment first.
I just don't trust 'em even if they are bound by law I don't trust 'em.
There is NO reason to trust them now. They are manipulating the tax code all the time. The difference is this - a flat tax affects every person in the US. Congress will have pressure on EVERY SIDE not to change those numbers until the 16th can be repealed.