Gosh this is ridiculous, why do I even have to explain it. If someone starts wasting fire extinguishers by emptying them out a window, do you proclaim that "if one thinks that the fire extinguisher can be put to a better use, they can start using them for that better purpose next door"?
The grid is not a battery; generated power in is always close to the current demand output. Any claims that crypto-mining uses "clean" power just indicates the claimant doesn't understand that power is patently fungible[3].
[1] https://en.wikipedia.org/wiki/Peaking_power_plant
It depends on the type of plant.
There's a concept in power generation of "base load", https://en.wikipedia.org/wiki/Base_load , which is the minimum usage. Some plants are expensive and slow to start up and shut down. If they're cheaper per kwH, then it makes sense to use them, but only for base load.
Then there's https://en.wikipedia.org/wiki/Load_following_power_plant , which are halfway compromises. They spin up and shut down perhaps twice a day. If load starts falling (or grows more slowly than predicted), these'll definitely get spun up less often. If load rises (faster than predicted), more of these will be used while plans are made to bring up more base load power plants.
Then there're peaker plants, https://en.wikipedia.org/wiki/Peaking_power_plant . These are expensive but responsive. They spin up and shut down within seconds of a change in demand.
This is all controlled by auctions, at various timescales.
A example that might be popular among the Musk fanboys of HN (e.g. me) would be https://www.theguardian.com/technology/2018/feb/23/tesla-bat...
So the power company has a grid, and they monitor the amount of power generated with the amount of power consumed.
They know this because they have sensors at the substations that can tell how much power is being used.
If too much power is being generated, they can sell the power to another company, or turn off excess capacity.
If not enough power is being generated, they can buy the deficit from another company, or increase the generation.
It's really straightforward.
In all seriousness, there are many types of power plants, the exact mechanism depends on how the actual energy is produced. It is true that for some renewable energy sources like e.g. solar energy, you cannot regulate production and need to store excess energy. But for fossil fuels, you (roughly) produce by demand.
If one thinks that the energy can be put to a better use, they can set up a shop next door and start paying slightly more per kWh than the miner, thereby destroying their margins.