This is the problem with open borders, Productive Flight (some refer to it as ‘White Flight’), but basically the productive class flees, leaving your area only with the dregs, now instead of cities, it is happening to whole countries.
Not to defend The Berlin Wall, but it’s understandable why the East Germans felt they had to do it, they were losing their best people. Of course, the better approach is to actually make your country so that the best won’t want to leave.
Mostly true.