i think it will be very difficult for the google to maintain its position for such a long period of time, and the way Bing is constantly improving it self, i think it will over take Google in the next couple of years.
77.27% is the latest percentage of votes in favor of Google. I believe with the number of Google's competitive employees there is no way that Google will be beaten by other search engine even after 10 years.
The Google guys are humans too, and all humans have their limits. Successors will eventually take over, like all kingdoms, and that usually determines if a kingdom will continue flourishing or deteriorate.
Google will be the best search engine on the planet. Yahoo and Bing may improved but I think that is not enough to defeat Google in terms of traffic, accuracy, and speed.