I question the premise that low pay caused this drop. PhD research was never about financial security , instead it pays in prestige and expertise of notoriously ramen-eating overworking geniuses. Prestige has certainly gone down since they became so commodified, and expertise can end with a Master's. Most PhDs are not even computer science and related fields (where the most interesting research roles are in companies).
We should rethink the duration and archaic formulation of the doctoral programs. Our times are faster