We have a certain tax rate that exists today because we have to raise a certain amount of revenue. Remember, the only reason we have taxes is to pay for things that the government needs to do under the provisions of the Constitution.
The revenue from the income tax is derived from applying a certain tax rate to the net profit of what people are making. The higher the rate, the more it will stifle economic activity and disincentivize earning more in order to avoid paying a high tax rate.
It seems that Pro Publica doesn’t actually understand how the tax system works. The problem with their latest analysis is that they argue high income earners are somehow getting away with something by not paying taxes on unrealized earnings or gains, but this is something completely different and should be treated as such. If your tax is based on net profit, as discussed above, that should be one rate. But if you decide that the tax should be based on gross receipts — you must actually make the rate lower because it is taxing a broader base.
In other words, Pro Publica is looking at the situation completely backwards. If tax collection is based on a base that includes unrealized income, the rate would be confiscatory. For instance, the death tax is already a double tax; you are paying taxes on income that you already paid taxes on when you earned it. To suggest that we should tax the unrealized gain on a death tax would actually be the equivalent to a triple tax — and from an equity point of view, it completely mocks the concept of fairness.