Electricity is just a vector of energy: it is neither sustainable nor unsustainable per se. It depends how it is generated and how it is used.
No ponzi scheme is economically sustainable; and BitCoin is no exception. Those who created the first quarter of the total possible bitcoin stock did so extremely easily. Every block thereafter gets harder and harder, The bitcoin founders thus held a worthless stock, but can increase their stock's value by getting other people to use bitcoin - they are always looking for the greater fool. It's therefore a commodity whose value depends solely on sucking more and more people in: it's a ponzi scheme; albeit an unusually clever one, with some very talented rampers and PR merchants now behind it.
Bitcoin itself has no value, so it is distinct from capital markets, where the stocks are of wealth generators.
It's perhaps closest to the gold market; but gold is tangible, has a little bit of industrial use, and some use as jewellery. However, unlike commodities (and unlike modern currencies), the technically possible total stock of Bitcoin is a known capped quantity, and the available stock is already very close to it. So there's very little adjustment that the supply can make to changes in demand. So any change in demand has a disproportionately high impact on price. Hence the need for early adopters to constantly draw in new people, to increase their stock's value.
There are always a few stocks in a market that generate no wealth themselves, but have sellers always looking for the greater fool. So although it's a mistake to say that Bitcoin is like all capital markets, it is true that within each market, there's often a worthless stock that floats up on belief and naive optimism to a high positive value for a while.