- No wall is big or strong enough to protect you from the consequences of fencing off the commons. That's because a nation that fences itself off from the rest of the world has typically already fenced its own people out of the best land within its borders. It has already impoverished its children and begun the path to internal collapse. There is no safety argument for raising fences.
- Immigrants, as well as your fellow countrymen, are trying to pay you rent for their use of your country. But this value was long ago securitized by your government and sold to private parties: These securities are called freehold land titles. They are held in a closed-end fund (land is fixed in supply) and trade, at the whim of existing owners, at a monopoly price (land is necessary for all life and production). Your economic trouble is caused by your government, not by immigrants; there is thus no economic argument for raising fences.
Here's a post I wrote last year that goes into another aspect of the economics.