like dig, i think it is the self-interest of the wealthy to ensure a society does not become too unequal.
this is the lesson of the french revolution.
if you want stability in the long run, you want neither an excessively flat tax code, nor an excessively progressive one.
the best slopes have a decent gradient to them. there's no absolute right or fair answer. but what people desire is probably as good a guide as you are going to get.