The thing is, societies that were close to ideal free market capitalism have done extremely well, whereas societies that were close to ideal socialism have generally done very poorly (in the long term). On account of local linearity, I think it's probably OK to use societies that were almost like what is proposed to judge the proposal.

The US isn't even close to a free market (although it used to be very close), so I don't think the current performance of the US economy is a reasonable basis on which to judge free markets.

Similarly, many people used countries that were barely socialist to judge socialism, which is also a bad idea.