I don't see how the effect could be zilch. Lots of interesting alterations, apart from the extension of the U.S. into a larger area.
From what little I've read, the peninsula didn't have a very large Mexican population until the in-migration of agricultural workers in this century. So you'd have had a demographic situation like California or Texas; less of a Native American concentration than Arizona or New Mexico.
With World War II, the West Coast got a rush of military, industrial and population development, and that would have had to include Ensenada and La Paz. And as the most pleasant areas around San Diego and L.A. got crowded and expensive, overflow development would have sprawled south along the coast instead of east to San Bernardino and Palm Springs from the 1960s on. I can well imagine flocks of retirees choosing Ensenada over Phoenix-- I know I would. Phoenix would have stayed the size of Albuqueruque. And with less of a real estate squeeze, so many Midwesterners would have moved west that Minnesota and North Dakota would be big nature preserves by now.
Environmentally, things would have gotten interesting as dam after dam was built, and water sucked off for irrigation, on the Colorado River. As it stands hardly any water reaches the Gulf of California, so former Mexican fishing villages are now miles inland. Somehow I doubt U.S. citizens, and local business interests, would take the Aral Sea treatment lying down. Though I guess it happened at Mono Lake.
And today the illegal border crossing issue would be an even worse situation for the U.S. to try to police. Look at the Haitian boat people's journey on a map, then give them 700 miles of coastline to shoot for a landing after less than 100 miles' journey. I can imagine the highway stop at the "Alta California" state line would be more involved than, "Got any fresh fruits or vegetables?".
Baja is gorgeous. I hope to see it again. Just because there are desert areas doesn't mean it's a wasteland.