I despise illegal immigration, but any talk of Texas, California, etc trying to secede is nonsense. As the Civil War showed, secession requires a great military force. The South had an outstanding army with able generals and still wasn't able to get it done.
Hispanics WILL have a political majority in those areas very shortly, but in the current American system being in the ethnic majority doesn't mean a hill of beans, legally speaking. The current situation of white Americans, legally disadvantaged by racial quotas, etc. certainly shows that.
Finally, I'm skeptical about how militant against white people the average Mexican actually is. The fact is, Mexico places the white citizens in its own country on a pedestal- most of the anchormen, actors, politicians, etc are white.
I think you are right, most Mexicans would welcome an American takeover.
1. The key difference is that in Mexico you dont' have political correctness to the degree we have here. The MSM and political elites will make sure the average Mexican understands the White Man has oppressed them.
2. Even if there is no civil war, perse, there will be a decrease in the quality of life (more crowded), while the cost (especially social costs) will increase. Sure, some goods and services will be cheaper, but the overall cost will outweight the benefits. Of course, the elites will still have their enclaves, so they won't care either.
Face it, the "Grow at any Cost" members of both parties have sold us down the river.