You're describing a scenario where the relationship between the countries becomes so hostile that we have to worry about what happens if one of us cuts off the flow of talent. But what we should really care about is making the flow of talent so voluminous and indispensable to both nations that relations cannot deteriorate to that point. That's real national security.
Relying on other nations is the exact opposite of national security and it's naive to think that they will be by our side when shit hits the fan. We've seen too much in the last hundred years to leave our most critical industries to chance.
Only importing immigrants who have political views that agree with yours? A country has a particular culture and set of views. Why would we want to import people from countries with inferior living conditions and follow the way they do things? There's a reason they left.
As for why let people in from poor countries, because we are siphoning off the most educated and ambitions people from that country, who bring their drive and good genes to our country. It's the same reason Asians, who consist of the top people in their home countries who chose to came here, do better in the US than blacks, who were brought here by slavers.
What part of the point "let people in from poor countries, but don't let them keep doing things the way they were doing them" is not clear? Sure they want to move here, don't let them turn this place into the place they left by making them learn the language and culture.
>It's the same reason Asians, who consist of the top people in their home countries who chose to came here
That is an incorrect statement. Most asians who first came to america were imported as cheap labor to compete against Irish unskilled labor. The Polish, Irish, and Italian who came to the states were definitely not the best and brightest, but because they assimilated and learned the culture prospered.
That has not happened in the history of mankind. Talent moving around is rather high on Maslov's hierarchy of needs. If your country is out of arable farmland, clean water, or minerals, you will start looking around for places to get those materials.
If they will not trade for them, generally you invade.
What does this have to do with anything? We're talking about resource wars. You can kidnap talent. Look at the origin of the term "shanghaied" and the British impressing American seamen.
> Today, that sounds like a much better option than a war of conquest.
You're not getting the point. Most nations can do without talent. Talent is willing to cross borders, depending on who's winning.
What nations cannot do without is food, minerals and water. Those are definitely zero sum games. Unless you're able to defend your goods with force or there is an abundance of them, they will be taken away from you.
Also, you're ignoring the national security implications of the thing. We cannot afford to be dependent on a foreign country for high tech skills. That's one reason why protectionism makes sense. As a country, we're willing to pay extra for certain basic manufacturing/services to stay in the county, such as aerospace repair and steel manufacturing, in case we ever become hostile with some other country.