The vulnerability of their business model was always obvious. What I don't understand, is why Google never attempted to enter new markets with different business models.
"What I don't understand, is why Google never attempted to enter new markets with different business models"
Google has always been attempting to enter new markets, but the new attempts amounted to a lot of moderately useful utilities, some questionably useful toys and curiosities, and a lot of misses. Mostly, the attempts seemed like random offshoots -- the products of a company that was supremely dominant in its position, flush with cash, and free to experiment in any way it so chose.
As I mentioned in a previous comment, it's very easy in hindsight to blame Google for having essentially rested on its laurels, squandering its lead and its position. And it's very easy to claim that certain threats (i.e., Facebook) were "always" obvious. They weren't. There were certainly some critical junctures at which Google should have focused its efforts on expansion in one or two directions. But Google was making attempts at expansion -- just not the right ones.
Harvard's Clayton Christensen famously described this situation as "The Innovator's Dilemma." There is a well-documented and almost axiomatic pattern, throughout history, of innovative companies rising to the top and then failing to predict who or what would eventually disrupt them. (If disruption were easily predictable, after all, it wouldn't be disruptive).
Here's hoping Google Glass will be successful.