Even with a wall, if he lets them stay, he's finished.
If he lets them stay and no wall. His supporters will actively work to boot him out of office too.
Letting them stay ensures war. So, they are leaving one way or another. It's just a question of when and how many of us get taken out in the process.
The wise axiom by Vox Day, "The reason we have homogeneous nations is that, periodically, people kill each other -- in large numbers"
Yup.