Compared to the United States, or Canada, Mexico is backward. However, compared to many European countries, particularly those in the east of that subcontinent, Mexico is ahead of them. Mexico would have a lot of developing to do before any full integration, but considering they have only a third of the American population, and are at the high end of the so-called third world (they are actually labelled as low-end developed by the UN), even if they were made part of the United States tomorrow (which they shouldn't be), the United States would not become a developing country.
It most certainly would lead to poverty and loss of income-way of life in this nation (and I am sure you agree:0,
EVEN SO The USA should be a sovereign nation appart from Mexico-Candada and any others. A plan for unified defense against say china- NATO style is OK, but not Political-Governmental-Societal Inegration!