Without question our borders should be secure. But every action has a reaction and if 20 million people would vanish from our country overnight it would cause a lot of problems and likely an economic disaster. Because of our governments failure this problem has grown but our society and economy has grown with it and as a result has assimilated these people into much of it.

Just off the top of my head I would expect:

The glut of immediate empty rental properties would cause a real estate crash.
Food shortages as a result of lack of labors in the fields, which would cause an immediate jump in prices.
Many small businesses closing up because of labor shortages and spike in wages.
Many layoffs across the board in many school systems.
The loss of whatever the annual spending and taxes paid from 20 million people is in our current economy.

Of course this is all really an exercise in futility because it never will happen, but the answer isn't for them to leave overnight but instead to control the borders and stop the increase, deport anyone who commits a crime and as time moved forward the country's economy would continue to absorb those that are here and if the increase was properly control with legal immigration the problems related to the unchecked illegal immigration would diminish.

The bottom line is that horse has long left the barn and can never be simply erased and undone in one imaginary sweep of a brush. Families in our country are continuing to have less children and as our population ages we need a strong leagal immigration policy in place or we will face a greater nightmare then we are already looking at as more people retire and draw from the system and less young people working and putting into that system.