So far in this book, recognizing cooperatives as business organizations, we have discussed the functional characteristics of cooperatives operating within a market economy and examined how cooperatives could be redesigned and restructured, if necessary, in accordance with the workings of the market system. This chapter reverses the question and explores how a market-aligned cooperative sector would alter the nature of a market economy. As observed in Chap. 1 , cooperatives have played an important role in each phase of economic development since the early modern period. They have also maintained a substantial market share in specific industrial fields. However, cooperatives have never been a predominant form of enterprise comparable to capitalist companies. The primary reason why capitalist companies have prospered and become the dominant form of enterprise in a market economy is that their organizational structures and mechanisms are well suited to the market system. Similarly, if the organizational structures and mechanisms of cooperatives were aligned to fit the market system, the cooperative sector might expand in scale and scope, consequently accounting for a greater segment of a market economy than it currently does. Based on the theory developed in Parts II and III, this chapter presents a thought experiment that explores how a market-aligned cooperative sector, and an extended system of business corporations encompassing both capitalist and cooperative firms, can affect the capitalist nature of a market economy. Specifically, recalling that cooperatives were originally established to counter capitalism, we examine whether a market economy with an extensive cooperative sector is a less capitalist society or no longer labeled capitalism.