Nobody I know has ever said poverty would disappear under the FairTax.
88% of the people live above the poverty line, which means that any increase in the tax rate hurts them more than the FCA benefits them. That is a super-majority voting to keep the rate from rising. Increasing the poverty-level definition would effectively require an increase in the tax rate on the remaining tax base. Again you have a super-majority to oppose such a re-definition.
Not gonna happen. If the rate starts at 23%, it will be down to 20% within five years, and 15% in ten years. Government spending correspondingly smaller.
Never happened in the history of taxation within the realm of unlimited spending.