You can’t destroy energy though. Where is it going?
Consider a closed system. That energy has to go somewhere. In the geothermal example, it is going into waste heat — heat which cannot be re-harvested because it requires a temperature differential.
If you don’t require a temperature differential, where’s the loss occurring here? How is the “waste” heat non-harvestable? I don’t see how a closed system could ever reach absolute zero.