[UweSchmidt]

It was just an example to clarify the criticism of "cryptocurrencies waste energy". It would be up to the cryptocurrency advocates to show such a function.

On the plus side, the problem does not necessarily be "proportional to D", in fact, finding a better function would be incentivized. Since the computer cycles would produce a "real" value (curing cancer), the upper limit for these coins would be the amount of money society would spend on a cancer treatment.

/speculation