"But why? Really, is there any fundamental reason we should expect Google to remain perpetually on top of the search engine business?"
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.
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.
While not explicit in the parent post, I think his major point was about a leader in a mature technology field not often being replaced.
Google supplanted Altavista largely because they made huge strides in a rather new field. While it's definitely possible, it's a little hard to image someone coming along with a web search that's impressive enough to supplant Google. It's more likely that "search" would evolve into something not entirely comparable to today's search, and replace them.
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.
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.