Can you prove it's not? At this point if anyone wants to propose an alternative then the burden of proof is on them. So far everything else that we've tried has failed disastrously.
I'm not making a judgement either way. You're saying that it's capitalism and multiparty democracy(i would question whether there's a difference between both the parties in the US, does it even count as multiparty?). China's the second largest economy in the world and there's no multi party democracy there.