This is a really great point, and probably the key one. It's also probably a good assumption that heat extracted from the Earth's crust after some point would

This is a really great point, and probably the key one. It's also probably a good assumption that heat extracted from the Earth's crust after some point would stabilize due to heat differential from the layer's beneath the crust. So we could take out some percentage of the heat of the crust, thus lowing the total temperature by some amount, but then that loss would be replenished from further down in the core. It looks like from the calculations elsewhere in the thread that it probably wouldn't even be physically possible to extract that much energy from the crust. We'd just run out since there would be some lag time in replenishing the heat from the core through to the areas of the crust.