Look, it's not like Texas and California are our favorite states, right?
If we give the four southwestern states to Mexico, we will essentially be giving Mexico to our four southwestern states, after a serious drop of two decades of the standard of living in those areas. The 66 million in those states will quickly dominate Mexico's politics. The new nation as a whole will suddenly be the 3rd or 4th largest economy in the world.
Our own national politics will not be unbalanced: we will lose four Republican senators, three Democratic senators, and John McCain. Purplish-red Texas will be offset by purplish-blue California. They should have Mexico's corrupt government up to at least Louisiana standards in twenty years.
Redesigning the flag would not be a problem: four rows of 7, three rows of 6 stars. No need to change the national anthem or America the Beautiful, though we'd have to rework This Land Is Your Land.
All the liberals will no longer have to be bothered by living in a hegemon, the conservatives can take over that huge pile of oil in the Carribean without American fish-huggers interfering.
True, the level of college football will fall off considerably, but that can be rebuilt.