A wall would not be necessary if the consequences of trespassing and invading were severe enough to deter illegal/uninvited entry into a country. If however, the benefits heavily outweigh any such repercussions and you have no wall that is a recipe for disaster. To me that seems to be the point. The main reason they don't want to actually fix the situation is they (yes, a cryptic THEY) want to collapse the current governments so they can replace it with a global government of their preferred design. IF this is their intention they are doing a good job in many places in Europe, and if we don't stop being pussies about trespassers in our country it will also likely achieve their desired goals in the U.S.

IF a country does not want to be very strict in the application of determent for trespassers then a wall while it won't stop it, would actually SLOW it. Not so much if it is a only spotty patches of walls like we've had at our southern U.S. border.