Gang problem will decline because drug traffic will decline; other crime will decline because there will be fewer users stealing to support their habits; prisons will be less crowded;
one thing i have not seen brought up is a fence or wall will help prevent border bandits from preying on illegals. without a sturdy fence there is an area called no-mans land where bandits and rapists freely roam just across the border and commit their crimes. if spotted when border patrol or police try and catch them they run south. It is a great deterrent to rock throwers also.
anyone interested in more info, read the book Lines and Shadows by Joseph Wambaugh its an eye opener about the no mans land that was in San Diego. read a few free excerpts see if its something interesting. it is non-fiction and dead on accurate.