By keeping the Southern Division which includes L.A. we isolate that area so it can be controlled then weed out the illegals in all the smaller states it becomes. End of the California problem.
Here's the kicker though. We don't let the coastal division leave completely; we make it a territory so it can't join with another country without first getting our permission which we would never give because it would leave us vulnerable on that section of coastline. You just know it will either want to join with Mexico or Mexico will make it but not if it's still a US territory.
Then, in a hundred years or so, we take Mexico and make it a provisional country that does not have US citizenship but is protected by our armed forces. Then after another 100 years or so and it is brought up to US standards we can make it a full territory because there will be no reason to migrate north for work but there will be reason for US citizens in the north to migrate south for various reasons including retirement in the warm climate.
That's what I call a Win, Win, Win. :)
The idea of a territory is a good one. These people have proven that they are incapable of governing themselves.
Also, don't forget the Monroe Doctrine.