But some fences were created to serve interests that no longer exist: Hadrian's Wall, for one. The fact that someone >centuries ago built a fence to keep the northern barbarians out of Roman Britain does not mean that it presently >serves that purpose. Someone who observed Hadrian's Wall without knowledge of the Roman Empire, and thus the >wall's original purpose, might correctly conclude that it serves no current military purpose to England.

At the risk of generalizing from fictional evidence: This line of reasoning falls apart when it turns out that the true reason for the wall is to keep Ice Zombies out of your kingdom. Chesterton would surely have seen the need be damn sure that the true purpose is to keep the wildlings out, before agreeing to reduce the defense at the wall.