Prosperity in Mexico is important to America, so I am worried about souring relations between the US and Mexico; our third largest trading partner.
When the president of Japan came to USA to meet with Trump, a Japanese automaker cancelled plans to build a factory in Mexico. If Mexico becomes more impoverished and unstable, the US will suffer. In spite of border walls, we live in an interconnected world.
We, in US, preach a gospel of prosperity that people's around the world emulate. This can become problematic if obstacles are thrown in the way of prosperity that other nations, such as Mexico, seek to develop. International instability results.
Maybe there are too many cars in the world, but future cars can run on green energy. Sustainable prosperity, in Mexico, is in our interest for political stability in our neighborhood. It would not be in our interest if Mexico were to become a failed state on our border. Even a wall wouldn't stop folks from coming in via the oceans, such as people fleeing Syria on the seas.