"But why? Really, is there any fundamental reason we should expect Google to remain perpetually on top of the search engine business?"<p>In technology, it is rare that a leader loses it's place in the market.  More often, a new technology is pioneered by a new company that makes the old technology less relevant.<p>IBM never lost the mainframe market, but did not dominate the PC market.  Google did not take over PCs, but dominates search.  There are many more examples like this.