The issue isn't so much that the rates vary with income. Arguments could be made that high enough marginal rates have a suppressive effect on both tax revenues and economic activity. That's more an issue of optimization. No, the real problem is with how the complexity of the tax laws allow lobbyists to game those laws for their benefit. gaming it by allowing for specialized deductions of exemptions or even varying tax rates depending one arcane requirements that benefit very specific industries or companies. Legislators allow for the complexity to encourage lobbyists to donate to their campaigns.
A flat tax or sales tax would only prove beneficial in the long term if it could remain simple. Once allowed to grow more complex the lobbyist game starts all over again. Taking money out of the system is best done by removing the incentive to spend money on the system.
