Which might be a good reason why not every single Bitcoin miner is using hydro. If the expected payoff of mining is big enough to outweigh the increased prices of GPUs and nonrenewable energy, then I might choose not to move my setup to be next to a dam.
The same market effects that apply to factories, research facilities, server farms, etc... also apply to cryptocurrency mining. We can't have our cake and eat it too -- if energy isn't fungible, then not every mining rig is going to be set up using the most efficient power source in the world. Not every miner is going to have the ability to just "choose" to use cheap renewable power, and if mining is still profitable where they are using the power sources available to them, then they're still going to do it.