Greenspan is off his rocker. The biggest problem with a national consumption tax that doesn't tax items that would negatively impact the poor is that in times of crisis people will stop buying items they don't need (FYI, I am not advocating that food and basic services be taxed by a national sales tax). This happened post-9/11, and the economy (in terms of sales of goods and services) took two years to recover. Meanwhile, states and municipalities that rely heavily on sales taxes for revenue took a major hit. Unless there is an outbreak of unemployment tied to such a crisis, the one constant about the income tax is that people will generally still pull in the same levels of income.
Fight the good fight.