Most companies are not well served to have the same CEO forever. Fresh blood, fresh ideas, and fresh perspectives are vital in a world full of imperfect people. That is one reason I think a flow of people between the different countries of the world is a good thing.
On the flip side, every culture has been concerned about being eliminated or marginalized. Look at the resistance and opposition there has been to the United States around the world. People of other countries don't want us to impose our culture on them. I suspect that most people in the US have similar feelings about outside cultural influences. We are comfortable in our familiar culture and we don't want to be forced to change because of a rapid influx of immigrants.
So we allow and encourage emmigration and immigration to facilitate the flow of ideas and information around the globe, but we do so with numerical limits to prevent any culture from being overwhelmed by fresh blood, thus killing the old blood.
Deciding on the right number of immigrants is not an easy task. I think we need to loosen up the current restrictions a bit and make it easier for people to immigrate the right way.
I also want to come out in opposition to a group called "Americans for Better Immigration." This group seemed to be pushing for some of the same things that I want, but after reviewing their website I concluded that they really want only a very small amount of immigration. To them, less is more. I see their view as unworkable and selfish. This is a wonderful country and I want to welcome in as many people as we can reasonably support. As we lift these immigrants up, we help the whole world with the rising tide.