The only way to actually ensure that no millionaire, anywhere, pays less than 20% on their annual income would be to essentially suspend the rule of law for wealthy people, and give the IRS power to seize income from rich people at will within some very broad guideline about fair shares. This strikes me (she said, with dramatic understatement) as a very bad idea.
Yeah, taxing the rich more would necessitate suspending the rule of law.
Let's take another look at a chart Barry Ritholtz put up a while ago.
Remember, we can't raise taxes on the rich because they would never pay them.