Here's the thing. You're saying in order to raise the amount of money the current tax system raises, a single sales tax would need to be 80%. All that means is the current tax system is equally as burdensome, it just seems less so because it's spread over different taxes. After all, in this hypothetical comparison, the government spending hasn't lessened.

The issue here is that an income tax punishes people for earning more money. That seems wrong, and seems not to provide an incentive for people to work harder and do better in their careers. Nor is that just a hypothetical, I know of at least one friend who turned down a promotion because the new salary would put them in a higher tax bracket and mean they actually took home less money.

A uniform tax for everyone is the fairest way to tax. Rich people don't use public services more than poor ones, in fact a case can be made that they use them even less. So why are they funding public services to the same extent as the poor? To help the poor? Why don't they help themselves by being less lazy? Many of the world's richest men started with nothing. Even God only helps those who help themselves.