Prior to WWII the United States was the world's leading power in terms of Science, Engineering and Industry - not Germany or the British Empire. The reason that Central European scientists fled to America (and not Britain) is because the United States had the scientific, engineering and industrial base to absorb them. Consider some of the major scientific breakthroughs to come out of the US leading up to and coming out of the war: Nylon, Teflon, Synthetic Rubber, Penicillin, Solid State Transistors, Microwave Communication, Information Theory, a Vaccine for Polio... These all would have happened with or without the war and the migration of German scientists (though adding John von Neumann to the mix probably helped move things along).