As I have pointed out before, people like to exxagerate. It may-might-could. They think it makes their argument more plausible. The problem with this is often the exxageration goes beyond common sense. For example, some exxagerate the number of illegals in the country and sometimes they use a number that is higher than the total hispanic population.
Same case in this post. Simply looking at the population of Latin America and the birth/fertility rates you see that the numbers are not plausible.
So let me set you straight.
Immigration rates will remain relatively high. In order to maintain the economic output, the population growth rate will need to be around 1%, or slightly less. Since the domestic birthrate does not achieve this, the difference will be immigration.
And as the demographic projections show, hispanics will eventually be the majority.
As for your phrasing, "how is this good for America", I will respond by asking you "how is the alternative good for America".