Nonetheless, it is worth putting some numbers to gain perspective:
1. World electricity demand was 24K terawatt hours in 2019 [1]
2. Mt St Helens volcano released 24 megatons of energy when it erupted [2]. That is 28 terawatt hours.
3. Thus, you would be adding 824 equivalent Mt St Helen eruptions a year in terms of additional energy extraction from the earth. Which sounds like a lot, but it really isn't for several reasons.
4. In particular, the earth is already radiating substantially than this amount of energy to the surface. "Because of the internal heat, the Earth's surface heat flow averages 82 mW/m2 which amounts to a total heat of about 42 million megawatts."[3]. That is 42 terawatts of continual energy loss from the crust/upper mantle to the surface. That is 16 times as much electricity as humanity uses -- and it is already be radiated to the surface.
My guess is that if we started extracting this from 20KM down and bringing that heat to the surface, then it would cause some increase in energy radiated to the surface, but it would also be concentrating that energy radiated to the surface at the location of the plant and potentially decreasing that energy from its slow radiation path to the surface through the rock above it.
Either way that potassium and uranium is going to decay. Either way, that heat will eventually make it to the surface and eventually be lost to space. The question is whether we can stand in the middle of that process and capture it for use. Our using of that heat and turning it into electricity -- ultimately still turns into heat and is radiated to space. It just is turned into heat when it is loses on the electricity transmission grid, when it is used to heat a house, when it is used to move a car, etc.
TANSTAAFL really doesn't intersect with the reality that stars burn and the earth decays whether we use the energy or not. Entropy comes for us all. We are just trying to be a step in the ultimate transition of all this energy to the heat death of the universe.
[1] https://www.statista.com/statistics/280704/world-power-consu... [2] https://science.howstuffworks.com/environmental/energy/energ.... [3] https://www.worldenergy.org/assets/images/imported/2013/10/W...