This whole discussion seems a bit confused. 50TW is the amount that does reach the surface naturally as far as I can see. The amount that escapes.

In the Earth itself there is some enormous amount, far far larger, with more energy being generated as well from radioactive decay

So it's not like some sort of oil situation where in 250 years we'll be at Peak Mantle, i.e. "The planet has spent millions of years generating this heat and we're draining it at an unsustainable rate!"