No, you're completely misunderstanding the meaning of that number. 50TW is not the power generated in the earth's center, and definitely not the "available geothermal energy".

It's the heat flow that reaches the surface. The newly generated heat (from radioactive decay) is a fraction of that (estimates vary between 20% and 80%). The rest is a loss of the primordial heat that has been stored in those billions of cubic kilometers of magma for billions of years. And this loss has been happening for billions of years and there's plenty left.

So there are no physical reasons why we could not extract 500TW or 5000TW from geothermal energy. We'd be depleting the priomordial heat much faster than before, but it would still easily last for millions of years.

Of course, whether the engineering to do it on that scale would be feasible, let alone cost-effective, is a different question.