I disagree with your assessment that "not one of [Google's] three positions is easily defensible in the face of determined competition, and I suspect it is more by luck (and inertia) than judgement that they have avoided this fate so far." There has been vigorous competition for search, including many billions spent by Microsoft on Bing. Microsoft literally tried to pay people to use their search. Microsoft continues to lose hundreds of millions of dollars per quarter in their attempt to gain search share. I don't know how much more proof you need to see that Google's position in search is easily defensible.
Of course, Google also literally try to pay people to use their search, to the tune of many millions in Mozilla's bank account.
Still, I think Google's main risk with search is that they have become victims of their own success: when you dominate the market as heavily as they have done for several years, everyone wants to be high up in your rankings, and people are constantly finding new ways to game the system. Even with all they've done about pseudo-legitimate but probably unwelcome sites like content farms and shopping comparison services, there has definitely been an increase for the past year or two in the frequency with which I try to look something up on Google and give up because what they're giving me back is completely unhelpful.
When you're stagnating like that, keeping up in the race against SEO blackhats but not really pulling away from them, the market is ripe for disruption by someone who does things in a different and (to users) more helpful way, just as Google basically wiped out everyone else with PageRank and the related ideas a few years ago. I'm not disputing that Google are still dominant today and no-one else is really giving them serious competition right now, but as I said before, I think that's more by luck than judgement on Google's part.
