At least it is still a fixed rate. The one thing I hate more than having an income tax is one where different people pay different amounts.
No matter how high the national debt is, production is a measure of how well our country is doing. The government can always print more money or just not pay off the debt, however, they can't arbitrarily create something like food or fuel out of thin air.
The problem about a graduated income tax that punishes those who earn more is that it impedes production.