The desire to exempt certain goods and services from taxation in order to appease special interest groups will, I fear, prove overwhelming to today's politicians. The Question of Progressivity Congress will never pass a non-progressive general tax. Nor should it. Many Austrian economists argue that marginal utility theory doesn't apply to wealth. Thus, beyond the issue of today's political reality, there is an issue of fairness that libertarians can note.
All taxes are unfair and unjust, but moderately progressive taxes are less unfair.
It is highly progressive. To see why, let's take an example. It might fly politically simply because it is progressive. Already there are calls to make the national sales tax progressive. It nearly goes without saying that adding progressivity to the sales tax would add a great deal of paperwork and compliance cost to the system.