I believe that the establishment Republicans want more immigration to prevent the population from declining. They are not interested in improving the economy such that white youth will start forming families faster because that would mean hurting their oligarch donors. (Democrats want more immigrants for political reasons). Establishment Republicans sincerely feel that social security is also at risk and want more workers contributing payroll taxes.
No one seems to be interested in finding a way to have a good economy with a declining population. Why? Because debt. Our economy needs to be bigger as a mean of reducing the burden of debt.