Economic theory has had a theory of natural monopoly for nearly two centuries. Natural monopolies in the economic model are utilities where it is too expensive to have two gas lines to a house or two electric lines. There are many examples ranging from railroads to fire departments.
In
1984 in a book I self published called Transaction Based Economics,
based on my experiences in the Briarpatch Network, I identified an
'ordinary' monopoly where the service was so good no other comparable
service was needed. My example was a local Briarpatch publication
called Common Ground that had a listing of all the local new age and
hippy related courses, classes and services. Common Ground was so good
it became an ordinary monopoly.
Google is an ordinary monopoly and so was Microsoft (when it captured the business market with reliable software and open access). Craigslist is an ordinary monopoly. Amazon is pretty close to being one, so are Adobe, Intuit and Oracle. The same is true of WalMart, CostCo and TraderJoes.
When a business is so good, so innovative that customers are comfortable, an ordinary monopoly arises. There is no strong reason to go anywhere else. After awhile it can actually be difficult to leave because the monopoly develops some economies of scale, in costs, in service and in quality.
In the case of Google, Google is good because it pays attention to what its customers search for and improves everyone's search. There is no reason to go elsewhere.
Any ordinary monopoly can fail to keep up with its technology and customer desires and it will fail; it will invite genuine competition.