While someone living in Dallas would see $200k as being very well off, think about a couple who lives in San Fran, or New York. 100k each is not truly 'well off' in those cities, so they are unfairly punished if they wanted to contribute what little money they have after living expenses to a roth.
Except that consumption based taxes encourage wealth hoarding, and are extremely regressive for families that spend most or all of their income. A better format would be a "average middle class household cost of living" adjustment.
We have to have a variety of tax bases (consumption, income, property.) Otherwise those with income/assets in excess of $500k can easily move money around to avoid paying a similar share to people who are less well-off.
