The concepts and categories currently forming the basis of the contemporary worldview about the development of economic systems do not allow formulating objective laws for nature and society development. The ability of enterprises and organizations to influence their own business environment is now recognized by leading management experts. At the same time, it is not paid enough attention in the dominant mainstream economic theories. The present study is aimed at substantiating the approaches to integration of companies in the competitive environment. The methodology applied in this study is based on a contemporary theory of economic systems representing economics as a complex of interacting, transforming, and evolving systems, regarded as an alternative to the neoclassical paradigm with its division of economics into macro- and microeconomics, as well as the concept integrating neoclassical, institutional, and evolutionary approaches. The paper presents an approach substantiating the development of economic systems in the course of transition from industrial to post-industrial society, which is supposed to be implemented on the basis of harmonic balance. It is finally reasoned that temporarily stable forms of nature, society, and enterprise are characterized by a «golden ratio» rule for their competing parts. It is shown that mutual integration of the company which is in a state of stable statistical equilibrium with unbalanced systems represented by the enterprises of industrial, post-industrial, and intermediate forms of development can be carried out only by merging. According to the «golden ratio» rule, the relationship between the parts of economic systems and their changes during the integration of companies may provide a key to further study of the nature of M&A effectiveness.