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.

Advertisers still pay for CPM ads based on what they're worth to the advertiser -- adding a million worthless eyeballs to the ad will work once, but the advertisers will note conversions fell and pay less for the slot in the future, for the most part. There'll probably be some lift from people who normally block ads but still might convert on an unblocked ad -- it remains to be seen if the lift will justify the engineering effort spent dodging the blockers.

Yeah, makes sense from that perspective. If your payments are based on eyeballs and not clicks or conversions, then absolutely I can see why they try to get around it...