Can you imagine what will happen to Mozilla if Google stops the contract?
IMO, it is just a strategy of Google. First make the Mozilla heavily dependent on it and later ditch so the Firefox can easily die
Firefox would probably take some pay cut and switch to bing. I bet Google is quite happy as is: the search engine is 80% of their commercial needs with a browser, so they're just outsourcing another IE competitor. Also, I bet they don't view this so ruthlessly -- else they wouldn't be as committed elsewhere with OSS projects and the releasing the Chromium version of their browser.
If Firefox switched its default search engine from Google to Bing, I wonder how many Firefox users would even notice? Power users would complain for a day, then just switch their search settings back to Google. But I think switching to Bing (or Yahoo) would restore some healthy competition to the web.