What scares me about illegal immigration

I try not to be a demagogue on any issue, but I'm as close to an open borders zealot as there is. Yet I'm realistic as well, and there's one thread running through this quilt that troubles me:

• As many as 20 million illegal immigrants reside here.
• As many as 90% of them are Mexican.
• Even I can do that math: 18 million Mexicans have left their country for greener pastures here.
• The population of Mexico is 80 107 million, therefore 22.5% 16.8% of their populace (and by definition a much larger percentage of the work force) has left.
• There are entire cities of abandoned wives and children, the primary export and economic output of whom are weekly cash remittances of U.S. dollars from their husbands and fathers. The flood of U.S. dollars will only make U.S. imports more affordable when compared to domestic goods, yet since so much of the workforce has left and productivity/innovation are both down, it will also become increasingly true that these relatively cheaper imports are relatively better-made. So what would you rather buy? Expensive junk? Or inexpensive quality? Mexican merchants will simply have no one to sell to, and nothing to sell in the first place.
  
• To avoid this odd collision of economic factors, more workers will leave, exacerbating the problem, and the downward spiral will continue.

It is no secret that societal collapse follows economic collapse. And the society in question happens to be our neighbor to the south. The spillover effect will be unavoidable. This way, one day between today and D-Day, Mexico will cease to be a net positive to the U.S. and will become a net detriment. On that day, the nature of the debate will necessarily shift. I do not think that day has arrived, nor is it close. But it is coming.

Of course the humanitarian way of looking at this problem is addressing the effect on Mexico and Mexicans, not on the U.S. and Americans. But either way, it is exactly the same problem.

UPDATE: I've just fixed the egregious typo in bullet # 5. Please read again.

SECOND UPDATE: ...And the factual errors in bullet # 3. Yeesh!