Fossil energy is just too cheap. If pollution, climate change and other negative effects were really priced in, Bitcoin miners would stop mining or switch to green energy, for example geothermal in Iceland. After all we know there are big mining farms there already.
There is no reason to assume developing economies suddenly stop using fossil fuels, so the idea is irrelevant.
Extrapolated crypto electricity trends are rising until the cost of mining reaches above payout, there is no "exponential curve until it consumes everything".
D'oh, I submitted secondary reporting about this yesterday <a href="https://news.ycombinator.com/item?id=18333353" rel="nofollow">https://news.ycombinator.com/item?id=18333353</a> because this one is paywalled.