Lawful permanent residency is generally the ultimate goal of legal immigration. So if you let a million people in - tourists and (genuinely) temporary workers aside - you have to assume that you'll be dealing with a million permanent residents, and, eventually, citizens, somewhere down the line.

In fact, if that were not the case, it would make things even worse. Imagine if you take a million people, and after a few years, kick most of them out, and only a few get permanent residency. Who would want to build their life around that?