I don't clearly see how a massive exodus of American scientists moving abroad could happen. While I understand that young scientists might find it easier to relocate, the decision becomes significantly far more complicated for couples, even when both partners are scientists. For other countries or regions to become truly competitive, they would also need to increase their investment in science significantly [1].
[1] https://www.wipo.int/web/global-innovation-index/w/blogs/202...
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