Does it have to dethrone Google to be a success? I don't think so. If they can take away 5 points in marketshare from from Google that is a huge success and will draw in a lot more money for Microsoft. If they take five points from Google, they are also likely to take away 5 points from yahoo and that is a huge gain. I think Bing is a good search engine, nothing revolutionary, but every product doesn't need to be revolutionary to be successful.
Also keep in mind Bing is a start. It's a change to the brand name timed with some fairly significant technical refreshes and some obvious ideological changes about search.
There is a lot more to the space of "search" as we move forward into the next decade, much of which Google, Yahoo and Microsoft haven't even tried to move forwards on. Hopefully the kind of technology and infrastructure related to search will start to be re-purposed for other uses.
