Can the US Allow Illegal Aliens at the Rate of Half a Million per Quarter?

What happens if the illegal alien stream continues at the same alarming rates as they are today? Can the United States economy handle this? Should the US completely close the borders from every location other than certified checkpoints? Could we eventually have an Open Border policy in 50-75 years or maybe sooner?

Some of these questions were recently raised in an online think tank and one member asked;