I'm going to try and repeat an analogy I explained to my eldest daughter about search engines and reducing advertising income.
In the early days of the www there were several libraries with quite a bit of useful information however the publishers of that information, although easy to find, couldn't find anyway to make money from it and, anyway, it didn't really matter to a most since their sites were, in general, company or enthusiast sites. No one copied other sites much, what would be the point of doing that?
As the years progressed a new type of library came along and everyone liked this library much more than any other since, quite simply, there was more in it, it worked better and almost everyone could find what they wanted to.
Over time the library became bigger and better, it even allowed anyone the opportunity, for a fee, to advertise their products in the library. This advertising was very successful and the library owners thought "Why not let the suppliers to our library have a piece of the action, we'll make more and it will also give them the incentive to keep developing their sites."
Up until this moment the big library's thinking was very good however it made several fatal flaws which would come to haunt it in later years. The library, in its greed, let almost anyone into their inner circle without hardly any checks whatsoever and that was beginning of the great library's problems and which will continue to be so unless they decide to have a very serious cull.
Why would they need to have a cull? Because the library's users are fed-up of having garbage and advertising thrown at them from every angle. Why should a user have to continually sort the wheat from the chaff when it is the library itself that has created this unneccessary concoction?
Part of what I'm asking is whether you truly believe that Adsense has peaked and is in decline or if it's stable and it's snafu.
The solution to much of the current SERPs mess is in Google's hands. There are millions of sites out there scraping and creating garbage yet remain there simply because once or twice a year they may reach the minimum payout level which pays for their domain name, hosting and a meal out.
Maybe I do look at things too simplistically, I do prefer the K.I.S.S. method since it's worked well for more than 40+ years for me, however I feel it needs a simple approach to try and get the library back under control otherwise it may become so overly-complicated its users may desert in droves.
Yes, AdSense could become good again for some of us but only if its management makes a determined effort to mend its errant ways and have a drastic, and it would need to be a very drastic, sort out, since, quite simply, libraries are not meant to be the depositories for garbage and enjoyed by villans.
</end semi-rant of Google AdSense history>
PS To Google: Please feel free to implement my ideas even if it were to mean me losing my account. I would far rather have a decent search engine than the current disaster.