The challenge of a "flat tax" is defining "income". It's not as easy as you think, and basic intuitive "fairness" will result in a lot of income being excluded. Given that today's graduated tax system results in top-earners paying lower percentages, a "flat" tax will surely be worse.
As for getting rid of the IRS, who will collect, administer, and audit the flat tax?
As for getting rid of the IRS, who will collect, administer, and audit the flat tax?