You can't "guarantee" anything in politics. Cain does not know what the composition of Congress will be if he is elected POTUS.
However, his plan clearly states that 9-9-9 is a first step towards the Fair Tax. A Fair Tax is what conservatives have been dreaming for decades.
Cain also wants the Balanced Budget Amendment with this. This will limit gov't spending to a percentage of GDP. Cain also includes a provision that tax increases must be with a 2/3 majority while tax decreases can be done with a simple majority.
There is not one person who can look at 9-9-9 and say it is inferior to our current, corrupt, IRS-laden tax system.
Bears shouting from the rooftops.