Keep in mind that the declining CPC is coupled with increased revenue, caused by the amount of ad inventory they're moving. So, your second statement is not supported by your first.
(disclaimer: googler, but not in ads or anything related)
There is a limit on how much traffic you can buy, there is a limit on how many ads you can put on your pages before those ads push people away. And when those work arounds lose their effectiveness (my guess is Q3/Q4 2016) then revenue will be flat year/year. If people decide that Google isn't a "Growth" stock, they will reprice their value based on current revenues rather than projected revenues, and call for the elimination of everything that cuts into profits. As the original article points out, there are many previous examples.
[1] Currently also Bing :-) but will no doubt change when the agreement expires.
https://cgviews.wordpress.com/2014/10/22/google-changing-ad-...
https://cgviews.wordpress.com/2014/10/14/changing-google-ad-...
https://cgviews.wordpress.com/2014/01/15/google-ad-changing-...
https://cgviews.wordpress.com/2014/01/15/googles-changing-ad...
The two negative forces this employs then is that if they bump their CTR and yet people who click don't buy, this further depresses CTRs as advertiser value the clicks less and less. And by paying to send them search traffic that money is paid if you search for "new car insurance" (very valuable) or "Pythagoras theorem" (basically worthless).
What is really spot on in the article is that when the 'luster' of being a growth company comes off, the stock feels it disproportionately. And as RSUs are the retention package of choice your "retention bonus" gets smaller as the stock takes a hit. That unlocks hand cuffs and people who are smart and aggressive opt to try something with more risk and a better potential reward.
It really is true that nothing is forever, but I know there is always the tendency to operate as if it is. Looking forward to the next wave.