As an American I grew up thinking of the Southwest as part of the United States, and of the Rio Grande as the natural border of Texas, so I don't have much to quibble about over the outcome of the war...but if the US had acquired the same territory without war, the people who were killed in the war would have enjoyed longer lives. Maybe one of them would have invented something useful, or written a good book, or had a son or daughter who would have.
And Zachary Taylor would never have become President...whether someone better or worse would have been President instead, I can't say.