The so-called "wall" is already being converted to a proposal for a mostly proposal for a fence.
If it's cheaper and the double fence is really helpful (the border patrol says it is) that's not really a criticism. The "fence" of 700 miles was approved in 2006 and for some reason, only 38 miles of it was built.
A fence, including the one built on the border, had and has an extremely limited useful purpose. Barriers like a fence are only useful in specific and limited situations. Unless they are constantly surveilled and immediate action can be taken, they are able to be defeated and penetrated. Surveilling small sections are possible where manpower is available, but large sections require a very large contingent of manpower to be able to respond in an effective amount of time.