Agreed! There would be no need for a “Fair” tax if the government stayed within its Constitutional Bounds. And, as you pointed out — income taxes would be WAY lower...
that is why the first argument given in support of this absuridity is the notion of revenue neutrality. It is premised that we are taxed at the right amount but by the wrong method.
The current method and this proposal are irrelevant. We need to be taxed LESS, that is the REAL reform.