What I don't like about this argument is that it vastly downplays corporations' ability to form monopolies and cartels in the absence of government programs to

What I don't like about this argument is that it vastly downplays corporations' ability to form monopolies and cartels in the absence of government programs to abuse. They are perfectly capable of using whatever means are at hand. We've had periods of much less-fettered capitalism. It did not result in greater equality, in fact it had the opposite effect. In regards to your last paragraph, you should realize that some people support redistribution not because they want rich people to give them money, but because they understand that capitalism is a flawed tool for tallying contribution to society, and a well-functioning economy needs a rebalancing hand. Likewise there are lots of people, capitalists even, who have arrived at your "success" scenario and yet still don't share your social darwinist views.