'ad "coverage" has dropped over the last few quarters, meaning fewer and fewer ads are being shown.'
I wonder how much of that is due to people using AdBlock and similar ad-banning systems? I personally removed Google Ads from my AdBlock blocklist, because I think that the non-intrusive text-only format they use is responsible advertising and I like to support that. But AdBlock blocks Google Ads by default, and the vast majority of its users wouldn't bother unblocking them.
That said, however, one cannot count blocked ads as lost revenue, since anyone using an adblocker is someone who isn't going to click on the ads in the first place.