You pay 9% with no deductions and 9% fed sales tax on everything you spend money on. Any allready taxed savings you have are now worth 9% less.
Those on the low end of the scale have to spend everything they make just to get by.
You are doing an analysis with a couple of flaws. First, they will not be paying the 15% FICA so they will be 6% ahead on income. Second, you are assuming that there will be no growth and no increased hiring and no more dynamic economy. Third, you are not taking into count that many goods and services will be less expensive since corporate taxes are lower. All of this will happen.
The FAIR Tax, which Cain also likes, has a provision to mitigate taxes on food and so on for certain lower incomes - but that’s way too complicated to get into on an internet post.
The bottom line is, you can’t pull a couple of features from the plan and apply it to reality exactly as it exists now. The whole reason for the 999 or any reform plan is to CHANGE THE REALITY AS IT EXISTS NOW for the better.