Locality Alone Forbids Most Theory Extensions

This paper argues that locality alone imposes far stronger constraints on theory construction than is commonly recognized. Treating locality as a retained structural constraint on admissible descriptions, it is shown that most proposed theory extensions fail before empirical or dynamical considerations arise. Extensions that introduce new interactions, symmetries, sectors, or degrees of freedom while claiming to preserve locality are structurally inconsistent. The analysis identifies three generic failure modes. Extensions either duplicate constraint imposition already enforced by locality, rely on hidden global dependence to maintain consistency, or illicitly offload enforcement onto existing structure. In each case, the extension fails to introduce coherent new physical content while retaining locality. The argument is conditional and eliminative. It does not assert that locality is metaphysically fundamental, nor does it evaluate theories that revise or abandon locality. It proposes no alternative framework and advances no new physical principles. Instead, it shows that if locality is retained as a structural constraint, the space of coherent theory extensions is far smaller than typically assumed. The result is independent of dynamics, empirical adequacy, and specific physical models. It reframes theory extension as a structurally non-neutral operation, narrowing the range of admissible theoretical developments on purely formal grounds.