Incidentally the last time [well, more like the 215th time ago, because this thing comes up like 10 times a day] this came up, I came up with a figure for US junk mail's CO2 footprint that was comparable to Bitcoin [US junk mail volume is something in the 6 million ton range, and the figures I got for paper were 3-10 pounds of CO2 / pound of paper].

I'm not bringing this up to dispute anything you just said, more that if you're looking to crusade against industries that generate a huge amount of waste to benefit a relative few, I think there's a strong argument for also killing junk mail along with PoW cryptocurrencies.