They each bring back home with them, not just their motivation, but also the knowledge, skills, experience and wealth they've acquired while away, almost all of the key things that are missing in Mexico now because it left and came here.
If, while enforcing its border and law, America is a good, strong and encouraging neighbor, then the potential for an "everybody wins" happy ending is very high for both sides of that border.
It's a big ass mess on both sides now, but it doesn't have to be like this.
What has been done unto US, can be undone by US, but...
We'll have to vote for Trump in November for that change to have a chance of happening starting next January.
Repatriating so many motivated people back to home to Mexico might save and revitalize that country. I believe it will.
Might even cause a change to political structure
But just think of the social consequences of sending so many Mexicans back to Mexico. The influx would have profound effects. Many of them would no longer be willing to tolerate mordida and all the myriad defects of that culture after basking in ours.
I draw a comparison to the Battle of Athens.
It could result in the toppling of the existing power structure., so yay for that.