That's not a very realistic scenario unless we truly don't do anything about our border. While a lot of children have been aborted over the years, a lot of the abortions have been from... shall we say... returning customers. Usually, when a woman gives birth, there's significant time between then and the next time she gets pregnant. There are a lot of reasons for this, both biological and social. There are obviously some women that get pregnant multiple times in short intervals, but that's usually a matter of trying to have a lot of kids.