External kink modes, believed to be the drive of the beta-limiting Resistive Wall Mode, are strongly mitigated by the presence of a separatrix. We thus propose a novel mechanism explaining the appearance of long-wavelength global instabilities in free boundary high-beta diverted tokamaks, retrieving the experimental observables within a physical framework dramatically simpler than most of the models employed for the description of such phenomena. It is shown that the magnetohydrodynamic stability is worsened by the synergy of beta and plasma resistivity, with wall effects significantly reduced in an ideal plasma. Stability can be improved by toroidal flows, depending on the proximity to the resistive marginal boundary. The analysis is performed in tokamak toroidal geometry, and includes averaged curvature and separatrix effects.