What if spablum doesn't prevent a person from eventually finishing their task, but lengthens their browsing/searching session, requiring a few reformulations? Up to a certain threshold -- when the person gives up entirely or tries a competitor -- such delaying tactics may give Google more AdWords and AdSense impressions and clickthroughs.
Next, as the quality of natural search results drops, the value of the paid-placement slots goes up -- for both advertisers and users. This is one giant hole in the argument that it's effortless to switch to another search engine, if it offers better natural search results. Lots of the value from Google searches now comes from the relevant ads -- only if Google were to let competitors run ads from the same pool would this switching cost be eliminated.
Finally, the quality arms-race is something deep-pocketed, heavily-PhD'd Google can fight indefinitely... but adds an immense barrier to new shoestring operations. Google, given its position, would be perfectly happy if search quality just hovers where it is (or even declines slightly) -- as long as the cost for potential competitors to achieve that same search quality, at scale, grows moreso than for Google.