I think what you see in Alabama and Georgia is an outflow of folks who originally settled in metro Atlanta or Charlotte in the 1990s/early '00s, who have now spread out to other parts of the South.
If the birthrate keeps falling in Mexico, they will be in very serious trouble long before we will because nobody is flocking to Mexico like to the US.
That is obviously possible. But why would someone leave Atlanta or Charlotte for "the country"? They stand out pretty good in these parts.... and generally Atlanta has been tolerant of them and nobody else is - particularly where it's mostly good 'ole boys, trucks, dogs, and dip.