The US will be a much smaller country without California, Arizona, New Mexico, and Nevada.
In fact, the Mexicans seriously underestimated us as opponents and squandered numerous opportunities to win the war because of it!
It came down to a grim, hard-fought military contest, not a pushover for an aggressive American military "picking on poor little Mexico." Winfield Scott, Jefferson Davis, and Zachary Taylor simply happened to be genuine military genii, backed up by brilliant gentlemen on their staffs! We out-thought the Mexicans, as much as we outfought them in key battles, which were often very close contests.