Core-collapse simulations in spherical symmetry across six progenitors (9.75 to 60 solar masses) reveal that instantaneous flavor equipartition below a critical density can either boost or quench neutrino heating in the gain region, independent of compactness or nuclear equation of state. The study employs mixing-length convection and lepton-number conservation, yet the assumption of instantaneous equipartition remains a strong simplification that omits full multi-angle transport and collective oscillations treated in earlier works such as Mirizzi et al. (2016) on fast flavor conversion. This mechanism resolves a long-standing tension: why some high-mass stars explode while lower-mass ones fail, directly affecting r-process yields and compact-remnant demographics. Earlier one-dimensional models without flavor physics (e.g., Janka 2017) systematically under-predicted explosion rates for progenitors above 20 solar masses; the new results show flavor conversion near the gain radius supplies the missing heating channel. Limitations include the spherical symmetry that suppresses turbulent SASI modes and the lack of progenitor-specific rotation or magnetic fields, both known to alter explodability in three-dimensional runs. The findings imply that neutrino flavor physics must be integrated into next-generation galactic chemical-evolution models to refine predictions of heavy-element enrichment.