Indeed, I'm surprised that people never mention this. Google probably has been working on this for a while now. Just bundle the ad and the video in the same stream and check timing to see if the ad has been watched or clicked on, same could be done for text ads, just serve them from the same ip as regular results, at least on mobile native apps. They're probably not pushing it for now as adblocking is not yet that much of a problem on their bottom line and they don't want to give hackers the time to work around these anti-adblocking techniques.