Google's per-ad revenue was down 6% last year. Ads are 95% of their revenue. They have huge data to see trends.
Just because Google is still profitable doesn't mean that they aren't seeing a near future of red ink. Say per-ad revenue goes down 10% next year with the same costs and their profit goes from $10 billion-ish down to $5 billion. And if this decrease is a trend?
Competition from iAd and Bing Ads, smaller screens that are harder to advertize on, and so on. It's easy to see a near future Google that has to spam ads or make huge cuts to remain profitable.
And I'd argue with more and more accessible technologies these behemoths won't have the luxury of "lots of time". And hopefully the large sums of money will mean less.