But wealth is not just money. Infrastructure, industrial base, technology sector and military / space exploration complex, domain knowledge (just think about how hard it would be for a new upcoming power to replicate the deep knowledge of all the people working for something like NASA), highly educated population with valuable skills (tech, finance, science), real estate (houses and flats in US are bigger than rest of the world).

For another nation to get to the level of US it would need all of these things and it would take decades and massive investment to build up all of this. Few very rich people taking their money and leaving for some other country would not be the end of the world.