The argument for allowing open borders is an economic one. I will present it as best I know it. I am in no way endorsing it.
If you look at Japan, Russia and Europe you see an aging declining population. The USA is trying to avoid that.
Economic growth requires a younger growing population. Young growing families work hard so they can buy stuff, like houses, cars, TVs etc. that in turn generates economic activity as well as taxes. Taxes remember are necessary to fund the welfare state. Without ever increasing taxes the welfare state will collapse. Since natural born citizens are not reproducing at the rate necessary to maintain the state and since the huge baby boomer generation is retiring the state requires a massive influx of young workers to avoid collapse. The only way to acquire this massive influx of young workers is to allow just about anyone that wants to come here to come here. Ergo open borders and weak immigration enforcement. Blame LBJ and the Great Society.
I can agree that there is an economic consideration for immigration, legal and illegal, positive and negative. However. the USA for a number of years has had immigration more tuned to political/social objectives than economic. My parents came to the USA from Russia at the start of the 1900s and there was both economic(for the USA and my parents) and political(for my parents). Your argument on it’s face would appear to represent an intent for allowing huge programs of immigration for the purpose of benefiting/bolstering a needed USA economy.Actually when immigration over the last decades is critically looked at/analyzed you will see most of it is political. Immigrants from south of the border for voting power. Immigrants from Eastern Europe for goody-feely reasons of political persecution. Immigrants from Islamic nations to prepare the USA for Sharia law. Immigrants from SE Asia to assuage a National feeling of need to help those whom we failed in a war. To argue that the USA did not have or does not have the native young population to carry on the economics and production needed by this Nation is a specious argument as far as for my take. The Nation does not need a bunch of freeloader families to be sustainable. Perhaps the Boston incident can be a lesson.