Fundamentally, the setup is that Google is blocking all ads (and all ad revenue) on sites that serve any ads that don't comply with a certain Google-defined policy. Google of course makes certain that this policy allows whatever ads Google wants to serve.

What is the most plausible outcome? That sites want ad revenue, do not want to take risks on losing it, and hence will only use Google for ads (because those will by definition comply with the policy).

It's true that if a site serves both Google and non-Google ads Google may end up punishing the site (no ad revenue), even if it means some revenue loss for Google from the now-blocked Google ads. But if the goal is to make sure sites do not do this mixing of Google and non-Google ads, that seems like an acceptable cost to pay to achieve the goal, from Google's point of view.