Commemorating the 10th anniversary of the late Professor Kim Soohaeng’s passing, this article reappraises his intellectual legacy of Marxist economics and suggests a direction for ‘firm studies’ essential in this ‘era of great transition.’ Kim laid the groundwork for the scientific analysis of capitalism in South Korea by completing the first full translation of Das Kapital, and disseminating theories on economic crises. Crucially, he introduced Marx not merely as a revolutionist, but as a ‘rigorous economist’ who meticulously analyzed contemporary economic realities, emphasizing the importance of realist research. Even in today’s landscape of artificial intelligence (AX), digitalization (DX), and green (GX) transitions, conventional economics continues to treat the firm as a mere ‘black box.’ By contrast, Marx’s Das Kapital serves as a comprehensive theory of the firm, covering internal production, value creation, and the mechanisms of circulation and competition. While Keynes and Schumpeter focused on the psychology or temperament of the entrepreneur, Marx identified how social relations, class dynamics, and competitive structures compel firms toward innovation and investment. Therefore, economics in this transitional era must move beyond abstract formulas, and re-focus on the firm as a practical agent. To achieve this, economics must break down disciplinary boundaries with management studies, and recover a ‘realist research method’ that critically analyzes the actual business world. Ultimately, the tradition of political economy left by Kim Soohaeng provides a powerful and enduring guide for the task of redefining firm studies amidst today’s global crises.