Well yeah, another problem is that there's always another cryptocurrency. Since there's no widely adopted coin (Bitcoin is closest), people jump to the next one hoping that they'll get in early before it takes off.
So the problem isn't that a given cryptocurrency takes too much energy, it's that speculators jump from coin to coin. I think that will settle down as well, and we'll be left with mostly serious miners looking for actual profit who optimize costs down with cheap excess energy.
So what we're looking at is kind of a worst case scenario. Bitcoin rewards halve every four years, and Bitcoin valuations are unlikely to keep up. Lots of cryptocurrencies are also switching to proof of stake. Both of these together should result in drastically less energy being used.
So I'm bullish on crypto energy usage falling going forward, even if it gains mainstream adoption as a currency (unlikely).