Throughout our history there have always been companies at the top of their game, and as well there has always been a company that seemly comes out of nowhere to remove it from its place. I cite Wal-Mart which overtook Kmart for example as the most recent one. And sometimes companies get too big such as was the historic case with Standard Oil Company which was eventually divided into 34 separate companies.
Google started out from nothing. It overtook Yahoo. In some ways it may be overtaking Microsoft. It's innovative and powerful and its at a peak in its history.
But someone will come along eventually to unseat Google, and I suspect the same could one day hold true for Wal-Mart as well. Google and Wal-Mart both know this and that's why they continue to innovate the way they do business. To maintain their place and make it harder for competitors to fill in the blanks.