Physical theories often characterize stability by identifying upper limits beyond which structures decay or collapse, while lower limits remain implicit or undefined. This asymmetry has traditionally been treated as a feature of physical law rather than of existence conditions themselves. This paper introduces Existence Threshold Band (ETB) Theory within the framework of Existence-Quantized Geometric Theory (EQGT), defining existence not as a binary state but as a bounded structural condition. An Existence Threshold Band is the parameter range within which configurations can both form and persist for a non-zero structural lifetime. The analysis demonstrates an intrinsic asymmetry between lower and upper thresholds: configurations below the lower bound fail to form and produce no observable events, while those exceeding the upper bound form but cannot persist, generating decay and emission events that dominate empirical datasets. This asymmetry explains why observations systematically cluster near upper stability limits. ETB theory provides a unified structural framework for formation, non-formation, stability, and decay across nuclear, atomic, and cosmological scales, while remaining compatible with existing physical theories as effective descriptions constrained to observable regions of existence bands.