Bitcoin is inherently sustainable. As mentioned the work to produce a given coin is adjusted to the total amount of work being done to produce a coins at a relatively linear rate. The profit in mining is constrained by the price of electricity. Should the price of electricity exceed the average revenue per unit time being created by mining, then mining becomes unprofitable. All that means is that the system constantly yields an equilibrium. It only grows when such growth is sustainable. And if the growth, somehow, turns out to be unsustainable - then the required amount of work to produce a coin would automatically be adjusted downward as the work input declines. There's no sustainability issue whatsoever. As for the power usage, I would argue that an extrinsic drive on individuals, companies, and even countries to provide ever more power capacity at ever lower prices to be something very positive - especially as we are entering into an era where renewables are reaching grid parity.