There's nothing mysterious about the decline.

There are too many publishers

There are too few advertisers (some have moved on, others are disappointed)

The economy is krap EVERYWHERE (worldwide) and until that changes folks will hang on to their money.

Forget the ad blockers, they were never part of the system to start with.

Remember the ad blockers because the rest of the folks are beginning to see value in killing off third party traffic that drives up their data plans.

And (ahem) it is highly unlikely that there are any unique, compelling, and sought after sites IF THEY ARE NOT A BRAND. Just a sad reality that there aren't that many great ideas and those have been scraped and duplicated/imitated ad nauseum in multiple languages.

The SE's (not naming any specific) are offering more "answer" than link, thus the user never visits.

Scare stories of malvertising (and these are real) might stop some from clicking ... even if they aren't killing js at the browser or running an ad blocker.

The fuzzy animals (panda, penguin, etc) have killed you ... but that is something done to one's self.

All of these, and quite a few more (my fingers are getting tired (wink!)) are among the reasons advertising in general, and google in particular since many rely on g solely, is trending down.

There are too many publishers

I never quite understand this kind of statement.

OK, I can see it on a CPM based ad placement basis .

But on a PPC campaign, especially with contextually or interest based placements, how can there be too many publishers/sites?

Too few sites I could understand, in a competitive market I'd guess that would drive the cost up.

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I'm not sure I agree with the too many publishers thing, but there are a ton of other things that all come to play for advertisers, publishers and Google. It's just not the same system it was and I don't see it going back.

Too many means too many slices of the pie and that applies to all aspects. The only one who doesn't lose is the middleman.

A niche with ten competitors is one thing. A niche with 10,000 competitors is something else. The odds in roulette might seem a little bit better. :)

These days we see some pretty low quality advertisers as well.

Too many means too many slices of the pie and that applies to all aspects.

That's only applicable if everyone gets an even slice of the pie. The number of publishers in the GPN is fairly irrelevant; it's where users go in the GPN that matters. Traffic and engagement are the real currency. A million new publishers aren't going to upset the cart much if their sites and/or SEO are rubbish and they can't attract visitors.

To put it another way, if you own a 'greasy spoon' and another 100 'greasy spoons' open in your town, you've at least got a fighting chance. But if a McDonald's opens in your town, best you start searching the job ads.

Ugly, irrelevant ads. A perfect storm of failure in my mind. I don't think much beyond that. Other suggestions might sound plausible but I look at the obvious first. Everything else is just an excuse really. Variables maybe. But if you put the normal ads from before on my website and make that ad relevant to my page content, I know I will do just fine. The ads are safe now and publisher earnings dropped a few notches in priority. The ads are so safe from accidental anything, they reek and stink like ads. Sprinkle irrelevant into the mix and you have a dose of suck right there.

I believe too many ads are becoming/have become rather intrusive, popping up/sliding in while scrolling or auto starting a video/audio ad. If a site has these kind of ads, I avoid the site altogether. There are also the sites with simply too many ads on the page which, not only slow down the page, but make it difficult to read. I also avoid these kind of sites. And I would wager than I am not the only one who feels this way.

(and that too many comes back to haunt. yet again!)

I believe too many ads are becoming/have become rather intrusive, popping up/sliding in while scrolling or auto starting a video/audio ad.

I've never seen those ads on my sites and never knowingly deployed them. I'm not even sure if they're available in Adsense. If I was to see one I'd be mightily annoyed.