Well it’s waste in the sense that you are spending lots of energy to calculate magic numbers, which don’t have any other value except to prove you own bitcoin. It is or is very near a tautology.
In contrast to energy credits in scifi which is something like a promise to deliver a fixed amount of energy, like how a dollar used to have an equivalence in gold.
Like, do you imagine some future spacefaring civilization going around and blowing up stars in order to calculate hashes for their currency? I can, but really only as something that an evil civilization or runaway automation would do.
Why not build a dyson sphere and sell contracts on the energy instead, to people who will use it to be productive? Why is it better to burn all stored energy you can find off as quickly as possible to generate numbers you don’t otherwise need?