The best argument for not worrying about immigration:

According to the World Bank’s 2007 Annual Development Indicators, in 1990 Mexico had a total lifetime fertility rate of 3.3 children per female, but by 2005, that number had fallen by 36 percent to 2.1, which is the “break even” point for population stability in developed nations. The large number of women currently in their reproductive years means that there are still quite a few babies, but as this group ages, the number of infants will decline sharply. If this trend toward fewer children continues, there being no apparent reason for it to cease, the number of young people in the Mexican population will decline significantly just when the number of elderly is rising. As labor markets in Mexico tighten and wage rates rise, far fewer Mexican youngsters will be interested in coming to the United States. Since our baby boomers will be retiring at the same time, we could face a severe labor shortage.

...

For all of Latin American and the Caribbean, a rate of 3.2 in 1990 fell to 2.4 in 2005, a decline of 25 percent.

...

The United States also has a fertility rate of 2.1, but that is the same as it was in 1990.

(HT: Will Wilkinson)

Of course, this doesn't mean we should change nothing. We should still increase skilled immigration, and better border control would be good regardless of how many people are crossing. The point is that the sky is not falling. We don't have to worry about whether our culture and institutions can handle current levels of Mexican immigration sustained into the indefinite future; current levels can't continue. As for our overall level of immigration, I find the following historical data instructive.