Do you not understand what the point of legacy preference admissions is? I will supply it here: Legacies take the place of the higher performing non-legacy candidate, not the equivalent one. Is this difficult to understand? Why?
The problem with saying legacy preference is to take the place of higher performing non-legacy candidates is that legacy admits generally perform above average at least at Harvard although it's probably true elsewhere. See here[1]: The average SAT score among legacy students was 1543, while it was 1515 for non-legacy students.
So it could still be for replacing higher performing non-legacies meaning Harvard targeted and rejected a bunch of people with even higher SAT scores than the legacies but I don't find that very convincing.
[1] (https://features.thecrimson.com/2023/freshman-survey/academi...)
However making everything domestically just isn’t really viable for any country any more. The scale of our technological civilization and the diversity of goods and materials it depends on is more that any one continent can support, let alone one country.
It would be possible to collapse that down to one continent if absolutely necessary, but it would be incredibly economically painful and the US would need to give up on a lot of non-essentials and other priorities to devote resources to duplicating capacity that already exists elsewhere.