Ayn Rand used to call capitalism the unknown ideal. It's much closer to the truth to say that a mixed, social-democratic economy, roughly half private and half public, is the unknown ideal, as opposed to the two extremes of Communism on the one hand and "assume perfect competition" on the other.

After all, the latter's what we're still taught in our economics textbooks, even if the former is now somewhat defunct outside of North Korea. Whence also the old joke about the economist who when confronted with something like social-democracy, says, well it might work in practice, but does it work in theory?