I have always stated that this wall was to keep americans in.

There are several huge events that are very likely that americans will be moving south of the border.

• Martial law. Police state. FEMA camps are ready to go, just waiting for the trigger. At that point, 'citizens' will be fleeing the country, accept now there is a wall.
• Ice age. We are at the beginning of a mini-ice-age. And a lot of people will be migrating south. Especially after the month long snow storms next winter.
• Economic instability. There are no jobs in The US. Basically, 50% of the people make paper for the govern-cement or file that paperwork. Along with automation, there are no jobs. And anything can break the system.
• Food shortages. The US farmland soils are depleted. This along with the mini-ice-age are going to see drastic food shortages.

Add to that, that Mexicans have been heading south in larger numbers than those coming north, the only reason to build said wall is to keep Americans in.