It's just a hypothesis at this stage and I haven't tried to quantify it. Reading through this and other forums some people seem to have collected some relevant data. I do know is that my ctr was far stronger three years ago than it is today and yet for my niche today there still exists a diversity of Adsense advertisers and compelling ad copy. I know people also attribute declines in ctr to ad blindness, but then again discussions regarding ad blindness have been going on since i first joined Adsense when my ctr was very strong. Of course one cannot discount ad blindness as an issue either as I am sure that there may be a combination of factors at play.