The problem with this argument is that going from most of south America to any part of central America would be a large economic jump. And we know that people who come into the USA illegally tend to go towards major hubs with more economic activity and opportunity. The amount of economic migration would increase for sure. And in a situation where we already turn around over 100,000 a month for attempting to cross illegally, not to mention the number we never catch, it's definitely not going to advantage American citizens to have open borders along the Mexico-USA border.

If anything it would disadvantage citizens.