[pikrzyszto]

Other captchas also waste (your and captchas' provider) electricity. For example reCaptcha requires tons of resources to track your moves to ensuring you're "not a robot". Sure, the data is also used to serve you ads but resources are still wasted.

[RcouF1uZ4gsC]

But as efficiency improves, the electricity usage goes down.

With proof of work, as efficiency increases, the work increases to keep on wasting the same electricity.

If electricity prices drop, the work increases to waste more electricity to keep the attack expensive enough.