There used to be much tighter rules on immigration and many fewer people coming even while the US was the largest economy of the world. So I don't think it has

There used to be much tighter rules on immigration and many fewer people coming even while the US was the largest economy of the world. So I don't think it has always been a superpower of the US. Japan also had the second largest economy for decades and a very tight immigration system. China will overtake the US on economy soon if it hasn't already and they have very tight immigration also. We also overproduce PhDs right now and elite-overproduction is something that seems to be very destabilizing.