You forget that Mexico will still be reproducing. We are talking about a 20 year period. Currently there are almost 10 million Mexican-born residents in the US and 4 million in California, according to the 2000 census, which probably underestimates the numbers. Also, remember that only about 60% of the illegals are coming from Mexico.
Mexico has a population growth rate of 1.16% or 2.42 children born per woman [compared to our .91% or 2.09 children per woman.] What makes the Mexican population growth rate so significant is that it has a net migration rate of -4.32 migrants per 1000 [compared to our plus 3.18.] The median age in Mexico is 25.3 compared to our 36.5.
The bottom line is that Mexico won't become "an absolute ghost country." In fact, it will continue to grow faster than the US and be able to supply even more Mexican citizens per year than it does now. Ireland used to do the same thing for decades. It is not a zero sum game, i.e., Mexicans entering the US doesn't lower the overall population, it just slows the overall rate of population increase.
Oh come on now. Mexico's entire population is only 100 million or so and that includes the 10 million or so that are already here. A 1.16% growth rate is not going to make that much difference. At that growth rate, even 20 years from now you are looking at under 130 million people. The numbers don't pass the smell test and the Heritage Foundation should be utterly embarrassed to put out such crap.