How is that different from what can happen to our taxes now? Saying tax rates could be increased is not an argument that is unique to the 999 plan.
What is unique to the 999 plan is that finally everyone, not just about 50% of us, would be paying taxes. So it would be a lot harder for congresscritters to raise rates.
We have no federal sales fax to raise, once it is instituted it is there it is there forever for politicians to play with. Taxes are immortal.