Sometimes, things are that simple.

In chemistry, there is a simple but profound law that all first-year students learn: that matter at high concentrations automatically diffuses toward matter at low concentrations.  In biological systems, it can explain a lot of what goes on.

It apparently also holds for social systems.  Consider – if Country A has well developed social support (welfare) systems and exists alongside Country B, which has very little such safety net systems, guess where inhabitants will migrate.  If you are poor in Country B, you are naturally drawn toward Country A – why wouldn't you be?  But one has to ask, can Country A's social systems absorb all of Country B's (and C's, D's, and E's) disaffected and still offer the same beneficent social support systems to its own poor? 

This is the very situation that the U.S. (Country A) faces along its southern border (and others).  Can the U.S. take in the poor and disadvantaged of all of our southern neighbors?  Could China take in all of Southeast Asia's poor?  Could Europe take in all the poor of the Middle East and North Africa?  But not simply take in.  Take in and maintain the same level of social support?

If the answer to that question is only up to a point, then a controlled border is essential to the social welfare systems of Country A.  In a situation of fully open borders, the poor in Country A will face a far more problematic future.

Note, however, that the situation for the wealthy in Country A is not substantially affected by the drawdown of social safety net systems in Country A.  Those wealthy don't need such social systems.  They're wealthy!

And this could explain the love affair of the coastal elites with the open borders policy of the Democrats, just as their parents weren't troubled by the Democrat-led issue of mandatory school busing in the 1960s-1970s – their kids (our current elites) were in private schools back then – i.e., unaffected by public school busing.