It's only waste insofar as you don't ascribe value to Bitcoin. It's not fundamentally different from any computing task.
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?
People use this argument a lot, and I don't get it. You're trusting that miners don't perform a 51% attack; you're trusting that the underlying crypto doesn't have shortcuts; you're trusting the wallet software. Some of these things are unverifiable, leaving you only with trust.
Because bitcoin miners would buy it and "waste" it
However the real point about bitcoin is it’s efficiency decreases as adoption or speculation increases, whereas another use of computation such as say Google search this is not the case and they are actively trying to make it more efficient.