Flat in dollars is regressive in utility.

"If we taxed the 'rich' at 100%, it wouldn't even pay the federal budget for one year."

Not that I support the policy, but depending on how you define "rich" it might. Per Wolfram Alpha (http://www.wolframalpha.com/input/?i=us+budget+%2F+us+total+...), the US budget is approximately 30% of total US income. That's going to need more than the incomes of the clichéd 1%, but not terribly much more. If you let us borrow the same amount we're currently borrowing (extracting current revenue by taxing the top at 100%: http://www.wolframalpha.com/input/?i=us+tax+revenues+%2F+us+...) that shrinks down to about 10%. Ignoring, of course, the very important point that those people aren't going to keep earning the same way when they don't see any of it.

The wealth is concentrated into the hands of a few. Higher tax rates for higher incomes makes perfect sense when basing "far" on "ability to cope with the loss".

20% of a low- to middle-income salary is a far greater burden than a 40% tax on someone with an income exceeding $400k.

There is an old saying in my social circles: follow the money. If you want to make money, you arrange to take it from the people with the money. Luxury cars, luxury apartments, gastronomic restaurants, and so forth.

Why not tax these people more? They aren't actually going to miss the money, and ensuring that there are more healthy middle class folks with disposable incomes will actually help the rich get richer still.

Social welfare and a universal single-payer health care system is actually a grand firm of corporate welfare. Throw a living wage into the mix and you can reduce minimum wages to zero. How much better will the outlook be for entrepreneurs and startups in that environment? No longer will volunteer internships be exploitation, no longer will labour costs factor into viability of a business, and you will have many more consumers with spare change to serve.

And most civilizations have come to the conclusion that progressive taxation makes sense. The wealthy benefit much more than in a straight line proportion.

Not that current taxation policy is anything but ridiculous, but flat tax isn't particularly "fair", for many values of fair. For example, the burden of paying, say, 20% of gross income is wildly different for someone making 20k/yr vs someone making 100k/yr.