What to stop the Congress from right now raising tax rates 20-30-40%?
The same fear you express here about 9-9-9 applies to an even great extend to the existing tax code.
It is actually harder to raise taxes with 9-9-9 then it would under the existing system. Since everyone pays, everyone knows they will pay more for ANY tax hike. Currently Congress can hid tax hikes by playing with deductions and loopholes or by targeting the tax hike to certain income groups. That power to play games with the tax code goes away under 9-9-9
They could. As a matter of fact they have. The highest (I believe) was 92%. But those percentages have come down and they do fluctuate.
Sales taxes only go up.