To continually place the tax burden on the high income group is disproportionate. Almost a penalty for doing well, IMO. Above and beyond paying more into the societal pool than any other group dollar for dollar, it is also the same group that builds hospitals, librarys, etc to the benefit of the other income groups.
The tax burden in a flat tax is on the poor. A progressive system attempts to place a more equal burden across income groups. How do you see a progressive tax system as a penalty for doing well when it is marginal? As for the group that builds libraries and hospitals? Not necessarily- libraries are usually public entities anyway...
But given the imperfect information and certain outliers on both sides, if you had the pick between accidentally overtaxing some poor or accidentally overtaxing some rich, which would you take? Seems like a no brainer to me...