"Overall, the Perry flat tax plan would be a boon to long-term economic growth and increasing the competitiveness of the U.S. in the global economy. While it could be criticized for not immediately replacing the current system with a simpler tax code, it does offer taxpayers the opportunity for "self-help tax reform." In this way, it allows taxpayers the choice between the current complicated tax code and a simpler, single-rate tax code.
From a political perspective, the optional nature of the plan frees politicians from taking on the special interests directly. These special interest provisions will decrease in importance as people gravitate to the new simplified tax system.
The plan will undoubtedly be criticized for not being revenue neutral in the short run. But it is clear that the authors put economic growth and competitiveness ahead of revenue neutrality. Considering the state of the economy today, that is probably a good decision."
The old code would wither away over time.