Another couple of reporting quarters will tell if the trend has been stalled, reversed or set in.
There's only so much SERP space left to monetize, then Google has to look at growth markets where it can continue to earn from ad revenue or other revenue streams. I'm sure it knew that growth would stall many years ago and they had it clearly written into their strategy.
One thing they probably underestimated is that if you squeeze a lemon til it's dry, there ain't no means to supply the drink. There's not too many folks i know making a fortune out of Adwords - even some of the biggest spenders. Ultimately, someone down the supply chain gets squeezed - and at that level, folks cannot pay. So i sense resistance.
Ultimately, Google is dependent on the free flow of useful content via it's services to put it's revenue model on, and i really don't think that will last forever, with all of the variations occuring at the distribution level. The focus is becoming too vast - bigger than China - i say.
Technology can move fast also - just ask Global icons that aren't around anymore and state of the art inventions that no longer occupy a place on earth anymore - just inside 10 years even. But Google has a lot of reserves left - but i don't give it forever in it's existing guise.