Ask people who lived in East Germany during the Cold War. A wall can keep people in just as easily as it can keep people out.
http://www.ibtimes.com/how-berlin-wall-fell-two-reporters-re...
The guards with guns were the real impediment.
At any rate, we are discussing surveillance technology, not a wall. And it has nothing to do with Americans legally leaving the US.
"We can't do blah, blah, bah, ... yet."
Technology is always advancing, in fact at an increasing rate, whether you like it or not. Trying to scare people about the technology is not the answer.