You might be joking, but I am not.
When put on a Kardashev scale, all these energy concerns seem pathetic. Terrawatts? Lol. Zettajoules of energy await us in the stars!
I agree that terawatts seem pathetic when you're thinking about even Kardashev Type I, but I would hate to see those zettajoules of energy spent on zero-sum competitions instead of goods and services that improve our standard of living. That's still true even if the competition consists of Bitcoin instead of the nuclear arms race, Wheel of Fortune, Democrats vs. Republicans, World War II, and NASCAR. I mean, at least Bitcoin and NASCAR don't burn cities, but they aren't building space habs either.
0.1% of our energy being spent on Bitcoin mining is fine, 1% might be a little worrisome, 10% would be a real wakeup call, 50% would be a crisis, and 90% would be ruinous. Is there a natural limit? Competition from alternative currency systems with cheaper transaction costs, maybe?