After all, Google ads tend to be among the least obnoxious, so if I were to pay for no ads (on the basis that I simply don't want to see ads and not because of a greater societal benefit of removing them as a perverse incentive) then I would want a very clear contract that says for X group of websites, I will get no ads and they will get revenue from my browsing. I don't want there to exist some group Y that still gets revenue from my subscriptions, even though they continue to show ads anyway. In that case the subscription is unfit for purpose; it might as well be called a donation instead.
It's also possible that Google intends to play both sides and offer higher-priced, high-priority ads that are always shown. It could just be the AdBlock mess all over again, which is the trouble with having conflicting interests.