I don't think this is their mentality, I think this is more like a myth that gets perpetuated ad nauseaum.
The problem with a search engine is that there's no lock-in other than brand recognition. Google won over AltaVista and Yahoo by being superior. I still remember the first time I tried it, it was so much better that it made an instant convert out of me, even though typing "altavista.com" was rooted in my nervous system and this was in the days before they were big, before AdSense/AdWords/AdX. And I could see it at Internet Cafes catching like fire, within mixtures of technically oriented and unsophisticated users alike.
And it can happen again. What pains do normal users have when using Google lately? Malware, content farms, "aggregators", too many ads. The only reason for why Google is still number 1 with a near monopoly is because there is no better alternative. DuckDuck Go is awful for me. Bing too. You may not notice how awful they are, unless you're living outside the US.
And I'm pretty sure they know that they can lose users fast. And once a significant chunk of users are gone, advertisers will be gone too. That's why Android exists in the first place, though distributing Bing as the default doesn't really help Microsoft, so having your own platform only protects you against walled gardens. And there's even a bigger danger therein. Google doesn't even have to lose users for advertisers to leave - Google already knows that the majority of clicks on all served ads are done by a minority and advertisers are increasingly aware of this fact too, as quality conversions are going down. This is because targeting is not so good after all and because users are increasingly fed up with spamy results and annoying ads that aren't targeted well.
I think their problem is that they are trying to solve this through algorithms only. The problem with algorithms is that algorithms can be gamed, you only need to find the ranking formula, which can of course be done through trial and error. It's a whac-a-mole game basically.
But for popular searches, like Firefox, they could have exceptions in there to propel those as first results. Is it not obvious that users searching for Firefox actually want Firefox, the browser from Mozilla.com, in spite of Mozilla.com's ranking? Is this against their policy or something? And now that they have Google+ accounts, why don't they add a "Report Result" button? If flagging email as Spam in GMail works so well, why didn't they do the same thing for their search engine?