Suppose a large ISP filters out all Google ad frames from all the websites/webpages being served to its customers [at network level], and replaces those with ads that it has itself sold. Now suppose that a nation's largest ISPs collude and carry out the same practice, with a common platform on which ads are bought and sold. This possibility seems real and seems like it would quickly and materially threaten ad-dependent non-ISP companies like Google. Customer consent for this practice can be obtained by promising revenue-sharing [ad revenue shared with both publishers and consumers].
It's to be noted that any argument that an ad-blocker installed on a user's Web browser represents a conscious choice made by the user whereas network-level blocking/replacement of ads by an ISP represents a choice forced upon users by ISPs [and that this violates net-neutrality] is an illogical argument if you mix one key ingredient - opt-in performed by users. In this case, the network-level blocking/replacement effectively becomes a case of the ad-blocker being installed "in the Cloud" rather than on the PC.