In my planning profession, we have a concept called "sustainability." Growth needs to be "sustainable" to be healthy growth. Otherwise, growth more closely resembles a rampant cancer, eventually killing its host. Successful communities plan and manage their growth to avoid this condition. Some communities may be desperate for "growth at any price" or promote "growth for growth's sake" as their highest priority. This kind of growth is not sustainable - its benefits are short-lived. In the long run, severe economic and social problems are assured.
Our immigration policies and lack of enforcement of our immigration laws result in the same cancerous, out-of-control growth. We have the short-terms benefits of cheaper labor...but at what longer term cost.
Looking back to the pre-depression years of the 1920's, this nation also had liberal immigration policies to facilitatae cheap labor. Then came the great depression, and the years following when we suffered through national unemployment rates of 20 to 30%. There was a backlash not only against non-citizens, but against recent legal immigrants. Many hundreds of thousands were deported back to Mexico as a reaction to the unemployment rate of US citizens. The rate of immigration was not sustainable through the inevitable ups and downs of our economy. Citizens required their government to take the painful action of massive deportation to correct our previous unsustainable actions. That "repatriation" program was not an act of an ill-intentioned or evil government. It was a mandate from the American people!
This graph shows the number of new immigrants in the period from 1900 to 2005. Note the peak in the period 1910 to 1920. The came the depression in the 30's. I wonder what role the unsustainable numbers of immigrants might have had in precipitating our "great depression?". The numbers declined in the following decades. But we didn't learn our lesson as the numbers of immigrants begins it's cancerous rise in the 70's to the present at an increasing rate.
This link http://www.cis.org/articles/2005/back1405.html provides the current numbers of new illegal immigrants entering this nation each year: 3.7 million. This is far, far from sustainable immigration growth, even if they were legal, which they are not. Those who promote legalizing this number of existing illegal aliens are looking for disaster! The "bleeding hearts" who insist on opening our borders and ignore reasonable immigration limits are clueless to the disaster they our courting via their unsustainable policies. Even if the majority of illegals wanted to be assimilated (which they don't) current numbers do not allow for assimilation. We will end up, not with a melting pot, but with a balkinization that will perpetuate social conflict for generations to come. That does not translate to sustainability.