Hmm, wait. How it works, then?
Let's say I'm paying 1$ to display malicious ad which mines something. BTC.
If amount of mined BTC is lower than 1$, the whole operations becomes unprofitable. Why should I pay 1$ to mine 90 cents via ad?
If people are running this operation on scale, it should be profitable. But then payoff is by definition good enough to support ad-free service. Instead of taking 1$ from advertiser, google can theoretically mine 1$ on my PC and let me watch video without ads.
What do I miss?
PS: Also, I wonder what other sites could be affected. Requirement is "user spending a lot of time on a page", so maybe blogging services with long posts, or shady sites with online sports broadcasts. Ugh, adblocker slowly becomes a requirement for modern web.