Progressive taxation is unfair, but the people who are being discriminated against by progressive taxation also received all the benefits of an unfair inheritance system. If everyone started with the same amount than no matter then the world would be fair and progressive taxation would be unfair, but in this world some people are born to rich parents and others are born to poor parents, and it is only fair to have the children of the rich pay more than the children of the poor after they grow up. The rich is inevitably made up mostly of the children of rich people, so taxing the rich is a proxy for taxing their children. Also by definition the children they would have would be children of the rich.
After all the rich kids went to good schools got a good education got all the benefits of their parents income, which is much larger than that of the poor.
This is just my opinion but I think if there was a hypothetical world where everyone started out equal in every generation than even a percentage based taxation (ex. flat 10%) would be unfair there. Everyone started out equally, so they should all have to pay in an equal amount to provide equal benefits to all, otherwise it would be unfair. The only real fair tax in this hypothetical world would be a head tax.