Yes, you could argue that. But a _nation_ could improve its proportions by restricting immigration to a subset of the _global_ population.

To a certain extent, this principle is applied today. But I imagine it could be scaled up to something akin to highly selective university admissions process (e.g. interviews, standardized testing, resume critique, references). If a country starts with sufficient desirability, this could result in a positive feedback loop -- an increasingly competent and productive society.

I agree with you there. Unfortunately, the greatest motivation to bring in someone outside the country is rarely for their exceptionalism, its usual about making/saving money. And at most institutions, its far more cost effecting to get individuals who are just sufficient and will accept a substantially lower compensation than their domestic peers. The individuals who are exceptional have loads of options/opportunities in their own homeland.

> The individuals who are exceptional have loads of options/opportunities in their own homeland.

That is simply not true. Most countries do not have the demand, willpower or money to sustain, say, a world-class research institution.

There's also the networking effect of wanting to work were there is already an established community.

Not necessarily. Average =/= median. But with a population as large as the US, they will be pretty close to each other.