Does the current income tax structure discourage people from wanting to earn more money?
No, it doesn’t. People who think that are seriously confused about how marginal tax rates work. Earning more money never means keeping less. There is no penalty for earning more money. If you get moved into a higher marginal tax rate, it means that you will owe a slightly higher percentage on the money you make over that cutoff. The money you make at the lower tax rate is still taxed at that lower rate. The top marginal tax rate in 2003 was 35%. That was the maximum that a rich person would pay, if they had no deductions. Rich people in the US pay the lowest tax rate of any developed country.
