Open borders were practiced twice in history. First were the American Indians and it didn’t work out too well for them. Next Mexico allowed open immigration into TX and the Gringos took it away from them.
Hey, it ended badly twice before, let’s try it again!
The U.S. was generous in giving American citizenship to the Mexicans who remained in the territory we took over in the 1800s. Mexico, I don’t think, would be that generous if they took over our areas.