What are these existing laws to be enforced while all the charade of legitimizing/upholding immigration laws is going on?
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.