Social Security doesn’t go away. FICA taxes go away and are replaced. 999 raises the same amount of revenue as all the current federal taxes combined. So you don’t need FICA the SS is funded through the 999.
What is good about 999 is that it is transparent, and it widens the tax base to include more people (including tourists and visiting businessmen), all at a lower rate. It is nearly impossible to evade taxes, it is so low most won’t want to try to evade it. And with a 9% corporate tax rate foreign investment will flock to our shores. Even if some smaller countries have lower rates this is America where investors want to be because of our history of political and monetary stability.
“It is nearly impossible to evade taxes”
High sales taxes create black markets in goods.