As long as the wall is manned and it's under pressure, it wil not complete. Expensions will always be made. For example, after the orginal Qin was complete, another layer was built a few hundred miles to the north. Or in the caseof the ming wall, whe ming was battling theorbats, the hexi corridor section and juyongguan was heavily strengthened and expanded (it eventually become a double layed wall a few hundred miles apart. While later when ming was under pressure from the manchus, the bohai corridor become a focus, and lots of fort being built forming the shanhaiguan-ningyuan-jingzhou defense network. This made the manchu have to go around the yan mountains through mongol area and attck from xifengkou and datong (which they are able to breech 6 times). The caused a frantic contruction in these section of the wall.