From what I understand a lot of power stations produce excess electricity at low demand periods because it’s too costly to completely shut a generator down. So they dump that excess into huge resistor banks. Why not use it to mine crypto currency?
Specifically, miners in China are often located right next to hydropower plants and buy energy from them when it is cheap.
This is because you can't exactly say that shut off a river, because it rained too much yesterday, and is producing too much power.
Pumped water isn't a great solution as evidenced by the fact that it has been around for a while and people don't use it right now.
There aren't enough batteries in the world to recoup any significant chunk of this lost/free/extra energy.
Power plants will sometimes even pay people money to accept the electricity, because too much of it has the possibility of damaging something.
If free energy needed to be expelled and couldn't be stored for some weird reason, it could be used to grind wheat, purify water, or run scientific computing to contribute to curing diseases rather than attempt to acquire speculative database tokens.
This is not "some weird reason". Thus is how the current power grid works in many locations.
If your theory was correct, then negative energy rates wouldn't even exist right now. But they do.
As solar and wind grows, we will use more pumped hydro, but there won't be / can't be e.g. a tripling of it.
https://en.m.wikipedia.org/wiki/Load_bank
So instead of a resistor use old CPUs and GPU and mine.