I happen to think "social Darwinist" captures the prevailing Republican philosophy pretty well. The point of the label, created by historian Richard Hoftsadter, is that a species of laissez-faire economics treated the market the way Darwinians treat natural selection — as the sole natural and correct mechanism for distributing rewards.I think that this line of thinking fits well into the paradigm of the "magic of markets". However, markets are man-made constructs and suffer the flaws of all such systems. For example, it is unclear that there is such a thing as a true free market without issues of fraud (because regulations are subject to capture by political forces) or force (because how do you fund police without taxes). Once you have a mixed economy, there are inevitably going to be winners and losers based entirely on who gets to develop the rules and who starts off with advantages.
Going back to a state of nature involves either starting from scratch (which nobody appears to be actually proposing) or trying to unwind a harrowing history of inappropriate use of force/fraud (just ask the First Nations about this issue).
Given that, I honestly do not understand the opposition to mixed economies.