"If you're from a disadvantaged background, but put as much effort as a person from an advantaged background, then you would make the same after-tax income."
This is clearly untrue. If I have $2M US in the bank, it takes me no effort at all to earn $40K/year. This is part of the rationale behind a progressive tax, that money begets money with little effort, like a snowball rolling downhill. That's why only a fraction of the wealthiest 5% started life outside the top 15%. They love to tell the rags-to-riches story, but the riches-to-more-riches story is about 20 times more common.