It does not matter how big or strong your company is to fail. With today's rapid technological change, companies rise and fall faster than ever before. Google and many others have a big challenge with the growth of mobile device technology and where to put their Ads, which is how they make revenue. Keep in mind, Desktop Computer/Laptops searches have traded places with Mobile device searches in the last 2 years... Here is a recent news story about the problem Google faces -- http://www.nytimes.com/2013/01/23/technology/google-profit-exceeds-expectations.html Here is a list of Top 10 failures resulting from Technology and wrong decisions. These companies lost their edge to technology... http://money.usnews.com/money/blogs/flowchart/2010/08/19/10-great-companies-that-lost-their-edge
I don't think Google will ever disappear as they have bought into other markets eg. Android, Mobiles, Youtube etc . . and have such a domination over the other search engines. What you might find is specialist search engines emerging for industry, science, IT etc. .
Yes, their search engine could become super crappy, but i doubt they will disappear lol. They have too much money, how can you walk away from billions of dollars? You can't! LOL
"The sun never sets on the British Empire" This is the quote of British Empire. No one taught Roman Empire and British Empire would fall. Same will happen with Google. Google is RUN by Humans, one day it will fall.
I don't see them falling completely off. there may be a drop in use because something else comes along but they've been evolving for years..