I'm not that concerned over whether "income" is taxed at a flat or progressive rate.
I am concerned about dividing "income" into what used to be called earned income and unearned income - the concept of economic rent arising from platform monopolies, barriers to entry (but also to exit, especially for individuals' choices), anticompetitive practices, externalities (like pollution and burnout, or benefitting from public infrastructure and spending), market cornering (such as in real estate, particularly since land is a fixed and unevenly valuable supply), privatized natural monopolies (like commercial banking that creates the money supply), and inheritances that source back to acts of violence (such as privatization by land enclosure, or income from slavery).
I don't think capitalism is a desirable form of government, but I think a progressive implementation of economic democracy is the way to solve that, not "taxation of income". In other words, I'm a left-libertarian.