If some Americans I disagree with want fewer immigrants coming here, then a more stable Mexico would encourage Mexicans and other Spanish speaking immigrants to stay in Mexico. Mexicans wouldn't immigrate to the USA, and Guatemalans and so on would stop at stable and prosperous Mexico rather than making the further journey to the culturally estranged USA. Instead of building the wall, we'd make Mexico a net.
What short sighted idiots. Chopping down a tree to steal the fruit? Our species is its own Great Filter.