If/once the environmental cost is priced in, all will be fine.
Once that is done, nobody would use this coal plant for mining bitcoins anyway, because it will be too expensive.
There are many ways to waste electricity. Bitcoin is a big one, but so is electric heating of uninsulated houses (still very common in the US): https://www.eia.gov/todayinenergy/detail.php?id=39293
=> All this will be solved by pricing in the environmental cost.
This is like seeing someone else light a cigarette on a plane, and deciding it’s harmless for you to smoke because someone else is already smoking.
(Edit: changed metaphor to match original)
