We have a sustainable economy, but what we also have is people who are very good at gaming the system, and no good strategies from preventing them from doing so. It won't matter what economic system we come up with, because there will always be people who figure out ways to take more than they contribute. Capitalism accounts, to some degree, for human nature. The alternatives we have come up with historically either are more extremely authoritarian, or depend on everyone acting altruistically, which immediately devolves into extreme authoritarianism.