Bitcoin mining off unused power at a hydro plant, the cheapest way to do it, already doesn’t hurt anything because there isn’t anything better being done with it or better ways to store it.
That's a very strong statement. If you unpack it a little bit, what you are saying is there is absolutely nothing productive ("better") that could be using that energy. I don't know if that's the case. Bitcoin mining might just be the most profitable thing to do, so nothing else was attempted.