You are correct. The border fence argument is to me very shallow. In remote areas, there would be more tunnels under it than in my backyard from the moles and gophers. It would be a huge waste of money. I say more boots on the ground and drones.
There are also vast parts of the border that are so tough to cross that a fence would be a minor additional obstacle.
However if we had an Israel style fence I could agree. Matt Bracken has an idea of what we really need.