Akira, don't worry, no one will ever replace Google, and Google will probably never close down. Why?
Well,in terms of the search engine, it's by far the most commonly used. I've never heard anyone say 'lets yahoo it' or 'lets live search it', but I've heard plenty of people say 'Google it'. So, as well as having millions and millions of loyal users, they also have money.
As long as the Adwords/Adsense programs keep running (and, seeing as they're up there with the biggest money making programs out there), the Google search engine will stay up and running.

