I wonder if this might not be a viable revenue model for "premium" services. IE: service is "free", but we're gonna use your CPU cycles for our own benefit.
As long as browsers had a way to distinguish crypto-mining usage from regular site functionality and is able to throttle it separately when the site opt-into the appropriate APIs, and in other cases detect suspicious levels of sustained CPU activity, perform site-specific throttling and warn users about it.
I'd much rather support content creation on the web using processing power than with my privacy and limited attention span.