That's because you haven't yet considered all the alternatives. You're locked in a stage where you're still fighting the tide. Once you learn to accept the difference between what you want and what is, then you'll be ready to deal with possible outcomes.
Consider what is commonly voiced on this board: build a wall/fence, deport all illegals, and cut-off all funding and enforce hiring laws to encourage the others to return home. Likelihood of occuring: -0- Result: 20m illegals today, 50m+ tomorrow.
OK, so here they are. Now what are we going to do about it? Accept a rump US with the SW lost to either Mexico or some sort of Aztlan? Or, show just a little glimmer of our forefathers' initiative who built and expanded this country through arms & treaties against the British, French, Mexicans, Spanish & Native Americans?
Btw, private property & contract enforcement aren't end results; they are a core basis of law from which all economic activity results. A Mexico under US rule of law wouldn't be a 3rd world country; it would be a poor province (much like the South was) ripe for improvement.