Ah, but that's not true -- as long as you have a tax based on income, the IRS will be needed to make sure all income is reported and taxes duly paid. This is true for progressive or flat income taxes.
The personal privacy issue is the main reason (among others) why I am a national retail sales tax supporter -- on an NRST, there is no information that needs to be collected about my income, because the tax is not based on income.