 Solution

The longitudinal polarization of a massive vector behaves as ${ϵ}_L^{\mu }(p)\sim p^{\mu }/m$ 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.