Yes CPM payment is based on impressions. But ultimately an advertiser will stop buying ads, or significantly lower the CPM, if they don't see click throughs (or better yet, conversions to product purchases, etc.) for those impressions. They keep track of the CTR (click through rate) and if that falls below a certain level they're going to evaluate why.

I don't work in ads anymore, but I get the impression that that industry must be under the impression that CTRs and conversions are in fact dropping because of ad blockers, because there's a rash of new startups that work in the space around detecting and getting around ad blockers.