I dont think we have any real evidence that cryptocurrencies are predominantly using polluting methods of power generation. The entire argument falls over if it is all renewable energy.
https://hackernoon.com/the-reports-of-bitcoin-environmental-...
Besides you seem to be missing one important fact about energy - it is very expensive to store much of it, which is why many bitcoin farms use up available excessive energy because that way it makes them more profitable and the grid more stable.
Carbon output, on a global scale, which is already hard enough to keep under control. We don't need yet another massive source of energy use, particularly one that gives us virtually nothing in return.
Bitcoin, like aluminum before it, can be used to "export" electricity from places where it's cheap and hopefully renewable, like a hydro power plant.