Reno_Chris - 4:11 pm on Sep 1, 2012 (gmt 0)
I certainly agree that there are many potential causes and effects swallowed up in the formative statement of this thread.
There are also many other strong reasons why a significant number of sites are generally trending down in Adsense income. Some have nothing to do with Google's conflicting interests in promoting or not promoting the Adsense program. Since 2003, Adsense has become far better known among website owners and while ad spending dollars have certainly increased as the OP pointed out, at the same time the number of participants in the Adsense program has also increased astronomically, so there is far greater competition for those same dollars. Over the years, many site owners wanting to increase their Adsense income adopted similar website design styles and similar user created content formats - because it was easy to do. Some of these styles have been depreciated by the G search team as low in worthwhile content through updates like Panda. Others have fallen victim to just too much competition from similar websites.
While there are many sites with generally down trending Adsense income, it is certainly not universally true.