= Solution
The <longitudinal polarization> of a massive vector behaves as $\epsilon_L^\mu(p)\sim p^\mu/m$ at high energy. Generic self-interactions therefore make amplitudes grow as powers of $E/m$, eventually violating <partial-wave unitarity> and destroying perturbative predictivity. Simply adding a Proca mass also spoils the gauge cancellations that control ultraviolet behavior.
The Standard Model obtains the $W$ and $Z$ masses through the <Higgs mechanism> inside a renormalizable gauge theory. <Goldstone bosons> supply the longitudinal polarizations, and relations among gauge and Higgs couplings enforce cancellations of the growing terms. In particular, Higgs exchange cancels the remaining $E^2$ growth in longitudinal vector-boson scattering.
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