The bill will be written with the appropriate self-limitation so that there is no more danger of it being abused than any other law that is written. Also, before 9-9-9 is implemented the 16th amendment is repealed so that cuts out the government bureacracy. Everyone will have skin in the game so the pressure to keep these numbers low will be HUGE. With a taste of Fair Tax, the transition to Fair Tax will be easier.
Plus, remember, this is a temporary measure, a bridge to the Fair Tax. It is a way for people to dip their toes in before taking the plunge. I think that people are afraid to go full in for the Fair Tax, and this gives people a taste. I believe people will insist on the FT, after seeing how the 9-9-9 works.
You can’t build in a self limitation into a law that binds future Congresses. What makes you think any Congress has the power to do that?
You also need thr 16th Amendment intact in order to implement one of the “9s” - the income tax “9.” You can’t repeal it and then pass a law that includes a Federal Income Tax. Without A16, you can only have 2 “9s.”