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There is a strange misconception that the Wall was ineffective at keeping armies out, but it isn’t true. The Wall as we know it today was constructed during the Ming Dynasty. And, sure enough, when the Manchus conquered China they did so not by *breaching* the wall, but by the abandonment of a section of the fortifications (Shanhaiguan Pass).
You say that the wall was ineffective because the entire length must be patrolled, and an army can attack whenever. But this greatly overestimates the mobility of premodern armies. They are not free to move anywhere, they are constrained along certain corridors, and the wall was constructed to control these chokepoints. And besides, it was not meant to stop large invasions, but rather small scale raids, which is primarily what China was dealing with.
As for your statement that it was about keeping people in, you vastly underestimate the size of China and project modern ideas of the Great Wall into the past. The Wall did not become a symbol of China until quite recently–during the Ming and Qing Dynasties very few people would have even known or thought about it–it was just a border wall. The vast majority of the Chinese population lived within the Central Plains region, untold hundreds of miles from the Wall. In order to “escape” China they would need to trek that entire distance, and largely without spending money. And why would they do that? Beyond China lay barbarians and other such unpleasant things. There was no underground railroad.
The movement that concerned Chinese authorities was internal, not external.