Because one nation making a great leap forward to more advanced weapons systems than any other always works out so well?
Again, if the "object" is to produce some "pinnacle society" that leaves the rest of humanity behind but advances some sliver at the maximum possible rate, then yeah, you're absolutely on the right track. But is that really the object?
I'm going to agree with Commodore here. While it's true that Einstein, and a bunch of other immigrants as well as some Americans, were key in the development of the Bomb, what else where they key in the development of? GPS for one, which is a really big thing for improving the lives of, not just Americans, but people around the world.
When we say that the genie cannot be put back in the bottle, that doesn't just apply to the world destroying genie. It also applies to invent the wheel genie. Technology has to come into existence before it can diffuse across nations. If allowing more top minds into nations with the best opportunities means that those nations get the lions share of the benefit in the short run, it also means that those benefits are available to the rest of humanity in the long run.
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Last week a girl who's father is an immigrant from Korea won a gold medal at the Olympics for the United States. How are we Americans not better off letting this man come to the US and have a family here?