Sheesh, where did you get all of this from? Are you Cuban or something?
The US was one of the best performers among major nations - 100 million survived out of 300 million; that's a third of their population. A much better ratio than China, India, Russia, or European countries. Cuba is the richest per capita, it is not the largest economy. Like how Argentina IRL has a higher per capita GDP than Brazil but Brazil has a much larger total GDP because its population is much larger. I highly doubt Cuba has a population of 100 million after WWZ, even counting American refugees (which are the real reason Cuba has become prosperous). Cuba's population is barely 11 million in RL, I wonder what you think their population is here.
You will notice that China's and India's populations are now less than twice as large as America's, rather than five times as much as they are in RL. So it actually makes even more sense in World War Z than in real life that the US would still be the largest economy. And both China and India still have far lower per capita GDPs than the US, World War Z is not likely to change that.