[mikeash]

Might be impossible, really?

Mitt Romney paid about 14% last year. I paid more than that, as did a whole lot of other common folk. That seems fairly conclusive that it's not impossible.

We can just look at income in a single year for the comparisons. Total wealth isn't really relevant to the question, aside from the fact that it's strongly correlated with yearly income.

Your other points are astute, but I'm just pointing out that this phenomenon does in fact happen, and isn't even disputed. Only whether and how to change things is in question.