I completely agree that the tax deduction system is needlessly complicated and open to gaming.
But that's a side point.
Tax Deductions systems are applicable to both with a flat rate system (everyone pay 30% income tax), and with a progressive rate (your tax rate increases as your income increases, with joe average paying 30% income tax). Assuming the rules of tax deductions being equal in both systems, we can disregard them from a comparison.
If the argument is to eliminate all or most of those potential tax deductions and instead just do a lower (but still progressive) income tax, I could get on board with that idea.