(If Bitcoin didn't exist, the hydro power would still be there and would be used for something else, therefore less fossil fuel would have to be burned in its place.)
That's not a good assumption. There are many power plants in China in the middle of nowhere unconnected to much. It's completely plausible that the entire bitcoin network could run on clean energy that otherwise couldn't be used for much else because it's too remote (geothermal in Greenland or something).
All this outrage is misguided, the market incentives mean miners go to where the cheapest power is, all the industrial scale mining is located next to large renewables sources in China, Pacific NW, Scandanvia/Iceland.
The largest ethereum mine in the world is 100% renewable.
