= Solution
At energy $E\gg M_X$, the longitudinal polarization behaves as $\epsilon_L^\mu\sim p^\mu/M_X$. A Proca theory with arbitrary interactions can therefore produce longitudinal-vector scattering amplitudes growing like powers of $E/M_X$, eventually violating <partial-wave unitarity> and invalidating perturbation theory.
In a gauge-theory completion, the <Higgs mechanism> identifies the longitudinal mode with a would-be <Goldstone boson>. Gauge relations fix the vector, Goldstone, and Higgs couplings. Higgs-exchange diagrams then cancel the leading energy growth of the gauge diagrams, as in longitudinal $W$-boson scattering, leaving amplitudes compatible with perturbative unitarity up to the scale where the full renormalizable theory itself becomes strongly coupled.
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