Well, the Mexican War happened. The per capita GDP of a Mexico that iincluded what was annexed by the USA, would be surely higher had there been no war, since the land is more arable. Part of the USA military strength, may be dependent on the apparent desire of the USA to be a large empire of influence over many countries. It has also been a source of strength in WW2 and to a lesser degree of "sucess" in other foreign affairs but often it has backfired and lessened the overall well being of Americans.and sometimes others. Does Mexico now or the Mexico with the Northern annexation have the desire for empire? Certainly not as much as the USA in either case. The military-inustrial complex (weapons companies) is one of the biggest factors in everything in the USA. Social programs, such as universal health care may be possible if the USA were not spending so much money on foreign interventions that have so little benefit for the American people and often very negative for the USA..

Any answer to this question would be preloaded with many assumptions. We simply cannot know what would or would not have happened if some historic event had turned out differently. But we can know that this was one of two wars fought by the USA that forced individuals into American citizenship unless they actively abandoned their homes and property and fled. And that this was done during a time when a large number of the American people did not want Europeans coming into the USA. A point that is often overlooked in todays immigration question.