Without an income tax, would the spending power remain at the lower and middle class? The housing market has shown that people are loose with their money and pay any price that they can finance. Likewise, all goods will become more expensive until people can only buy the same things as today.

Income tax can be seen as a tax on companies that can't be embezzled. Most people are honest and want to see their full income on documents so a company has to pay the full amount of the taxes.

This leaves the lower and middle classes with less money, but it also forces the market to offer the goods for less.

The alternative would be taxing the companies directly. People would earn more, but companies would also request more for their goods because they have to pay more taxes. But would the tax payout be the same if companies have an even bigger incentive to hide their profits?