Continual advancement of technology will increase output over time...this has been proven over and over again. Someday machines will make machines with very little human involvement in the production process. It's already happening.
Moreover, and I think you might agree, there is a theoretical limit to which our planet can support our global population. Resources are finite and even with our technology we human beings still are subject to the laws of nature and cannot crowd out other life forms we are dependant on for our survival--without consequences. Until we progress our technology to the point where we begin colonizing other planets in the galaxy there is a natural population limit (somewhere) to our species. For all we know, we might have already exceeded it, and nature is in the early stages of addressing it.
I agree with you that the political will to address the immigration crisis is lacking and little will be done on this front unless and until the American People demand a change and hundreds of politicians lose their jobs over their stance on this issue. Stay Tuned.
But that was exactly my point. What is the purpose of increasing output over time if my customer base is stagnant or declining in size? The "increasing output" model for productivity growth relies as much on a growing consumer base as the welfare state does!