For those against a wall at the border, ponder this. Whether it be a wall, or a standing Army or a million miles of technology backup by an Army, it will cost many many billions. The alternative is to do nothing and the cartels of central Mexico and other lunatics from who knows where will have free reign throughout this country. Then what?
From roughly 770 BC to the 1600s, some folks who built walls not only expressed their territorial intent and desire to not be invaded, they left that legacy carved in stone. Though those fortifications were modified and reconfigured during that time, they stood as a barrier, a border, and a legacy for those who would follow.
https://www.chinahighlights.com/greatwall/history/No matter how many troops had walked that boundary, nor how many eyes had scanned the lands beyond it, nor how few might, with the vagaries of time and leadership, have patrolled that border, the wall stood as an enduring statement that the land inside it was theirs, and for invaders to stay beyond it.
Had the wall not been there, all that would have been left for posterity would have been well worn footpaths, some scattered campsites and detritus, and the ongoing expenditure of treasure, manpower, and, in the end, political will to keep invaders at bay.
A virtual wall will leave virtually nothing if the vagaries of political intent remove funding. A physical barrier will endure, can be repaired or modified, and will continue to exist--and more likely, so will the country within it.