Actually it was a rhetorical question :) The answer is none. Google became popular by word of mouth alone, as opposed to Microsoft who spend tens of millions of dollars trying to fool people into buying their shoddy products. I have sworn by Google since 2000 ! Back then it was already the best search engine nobody knew about.
I understand with most things, there is an installed base, and people are resistant to a new learning curve, but if someone eventually does make a better search engine, it will be absolutely free to use, and Google could be abandoned overnight. If somebody really came up with something that would cut out the crap that Google produces (and all the other search engines produce even more of!) then I'd use that engine in a New York minute.