Strong coupling in string theory: physical irrelevance beyond low-energy supersymmetry

We examine the three principal strands of the strong-coupling program in string theory developed during the 1990s---S-duality, M-theory, and F-theory---and show that their most controlled formulations rely on supersymmetric regimes, typically involving unbroken supersymmetry at low energies. These results are obtained within narrow, highly constrained corners governed by supersymmetry. In the absence of experimental support for low-energy supersymmetry in the post-Large Hadron Collider era, the physical relevance of these regimes remains unsubstantiated. Without this structure, the computational control underlying the program is lost. It then follows that the central achievements of these frameworks lack a demonstrated connection to real-world physics.