Keeping illegals illegal while forgetting to deport them just happens to be the sweet spot of a strategy to keep a whole class of people working for sub-par salaries with no bargaining rights and no social protections like unemployment insurance, workman's comp if they get injured, no social security, no withholding and so on. Illegal employees are a huge gravy train for the employer class, both Republicans and Democrats. They are an underclass of super-exploited employees.
At the same time the presence of a large class of employees being paid sub-par salaries keeps constant downward pressure on salaries for everyone else. Which again improves profits for employers at the expense of their employees. Every politician who talks smack about the illegal immigrant "problem" is lying to you to your face. The two political groups perpetrating this immense fraud on the American public are the Democrats and the Republicans.
Donald Trump is in effect calling the Republican Party on its perennial hypocritical lying about being against illegal immigration when in fact they are all for it because they make so much money off it. Of course he is lying too but he is upping the ante on the rest of the Republicans who will still be in Congress trying to maintain the system long after Trump is gone.
Maintaining a class of millions of illegal immigrants is the edge of the sword of class warfare in the US.
Let's say Trump were elected and not assassinated and actually made good on his campaign promise to deport the illegals. (Because American politicians always keep their campaign promises.) There would be major dislocations in the US economy as agriculture and construction both collapsed. Service, manufacturing, and other industries would contract sharply.
Prices would shoot up because of labor shortages and because employees would no longer be as disposable as fast food wrappers as they are now. Salaries would rise and inflation would return. Interest rates would rise to accommodate inflation. Rising costs of borrowing would slow the economy and profits would fall or vanish. The economy would contract.
How do you think the economy can expand so much as it has in the past few years without causing any inflation? Because the presence of the illegal immigrant class keeps salaries from rising so labor costs remain flat while productivity and profits both rise.
What would be the effect of sending 17 million unemployed people to Mexico, a country whose government can barely maintain itself in power as it is? Between Mexico's revolutionary traditions and its modern cartel warfare, the influx of 17 million angry unemployed people could easily lead to the collapse of the Mexican state and the descent of Mexico, a country of 140 million people, into anarchy. Imagine that on our border.
An example of what happens was the mass deportation of 300,000 Palestinians whom Kuwait kicked out for siding with Saddam Hussein during the Iraqi occupation. The return of those hundreds of thousands of unemployed men to Palestinian towns and cities led directly to the Second Intifada - a year of rioting and guerrilla warfare against Israel. Imagine the same thing, but 50 time bigger, happening in Mexico.
Trump, in his own way, is pointing out on what rotten foundations our economy stands.