Without that influx, this concentration would be impossible. If the US heavily restricts immigration by software engineers, the industry will ultimately relocate elsewhere. Europe would love to get all its software engineers back.
Yes, it does, you used a figure to suit your argument, now people are rightfully asking for a more relevant figure to see if your argument still stands. So what % of developers live in USA? The rest is just purely your opinion.
More like you don't want to know, or it would turn your argument into an absurdity.
> At what thresholds would it make a difference one way or the other?
Since we can't know how many people are employed as developers, we must guess. But we can reasonably guess that if only 4.25% of the world's population lives in the US, it can't possibly have all the best developers, or even the majority of them.