It appears the seeds to the decline of web domination of search which Google commands today are in place, and have been for some time. Primarily this could be attributed to:
A) Web 2.0 has developed several major hits which provide primary sources of information (twitter, wikipedia, facebook), completely skipping the need for a search engine to find a websites. For a large portion of the past 10 years people didn't know which sites they would want to use, so they had to use Google to find them, now they have.
B) Search alternatives such as: Bing.com , Cuil.com , technorati.com, all combined will most likely never equal half of what Google.com has today in search traffic. But as they grow Google will decline.
C) IBM, Microsoft, Intel. All large technology companies who underwent (or are currently undergoing) major alterations in how they make money to combat competition and avoid obsolescence. My guess, 1 to 2 decades from now, Google will have undergone something similar to what these companies went through.