I don't think that it is sustainable at this current stage as most of our energy generated is from fossil fuels. I think when we start using 100% renewable energy sources (hopefully that day will come soon) then the only problem will be the cost of electricity and if bitcoin is going to grow enough to cover your expenses @janusface
Thanks as always @arckrai. My focus is one the cost of mining, i.e. electricity consumption. Fossil fuel or not is of course an interesting topic:)