The public decides if it is a Google Killer, not the software maker. People will use your software in ways you never imagined. The makers of one of the slide show websites said she was really shocked when Preachers became their most popular demographic of users -- they were uploading their sermons, who would have guessed that?
That being said, most people who go to Google aren't looking for the kinds of answers Wolfram Alpha provides, so I doubt it will kill google, though if it gets out of Wolfram's control it could begin putting out the kinds of results googlers want.
