Based on the "Great Tao Model" proposed by the Jiqing Zeng’s team—a framework rooted in classical physics, with the Yin-Yang model of elementary particles and the theory of existence fields at its core, and embodying the essential characteristics of a Theory of Everything (TOE)—this paper conducts a systematic analysis of numerous important attempts to pursue grand unified theories in the history of science (such as the Standard Model of particle physics, superstring theory, and loop quantum gravity). The study reveals that the fundamental reasons for the failure of historical grand unified theories lie in the following aspects: deviation from first principles, mistaking derivative phenomena in quantum mechanics (such as counterintuitive probabilistic interpretations and wave-particle duality) for essential laws; succumbing to mathematical formalism, where redundant assumptions (e.g., supersymmetry, extra dimensions) are added to cover up the lack of physical essence; and severing the intrinsic connection between fundamental physical quantities (charge and mass) and elementary particles, leading to fragmentation of the theoretical framework. In contrast, starting from the essential definition of elementary particles (electrons, positrons, and substons) and the inherent properties of the two fundamental physical quantities (charge and mass), the "Great Tao Model" uniformly describes all physical interactions and cosmic evolution through the theory of existence fields. It achieves a true grand unification that is logically consistent, formally concise, and highly consistent with experimental observations, providing a new path to resolve the dilemmas of historical theories.