AFAIK the Privacy Pass protocol has nothing to do with proof-of-work. It's just a way to allow users to solve a CAPTCHA once and re-use that single solution acr

AFAIK the Privacy Pass protocol has nothing to do with proof-of-work. It's just a way to allow users to solve a CAPTCHA once and re-use that single solution across multiple sites without jeopardizing privacy. Reduces the number of CAPTCHAs you see, but doesn't eliminate them entirely. What you're talking about is more like what Hashcash does, where it essentially replaces CAPTCHAs with a cryptocurrency miner, such that bots become more expensive to run due to the amount of energy they consume. The downside is it's not great for battery life for regular users either.