This is an interesting thought that crypto (or at least proof of work crypto) is backed by a sunk cost of an otherwise useless task.
But let me extend that paradigm: why is (or isn't) gold also not baked by a sunk cost? What was the point of all that expensive mining and digging if not for the sole purpose to extract that shiny metal from the earth's crust?
To some degree they are similar. The one difference is perhaps that BTC mining income is effectively guaranteed, ie if you join a mining pool and invest enough money, you will get a return of known quantity. In contrast, you could invest a virtually unlimited amount of money in a gold mining operation but not get anything back.
