It doesn't have to vanish to get taken over by executives that don't see the value in providing such a huge chunk of money to a company that offers a competing product. In fact, I reckon that most business schools teach this kind of support for your competitor is extremely bad. We know that brilliant and awesome companies end up in the hands of MBAs and it's all downhill from there. Mozilla should certainly plan for this eventuality.
You're way oversimplifying it. Google gets a huge influx of traffic through this deal, and Mozilla would most likely be able to generate income from their 30% of the browser space if Google didn't sign this (possibly even from Bing, which is even worse for them). That kind of market space itself is a valuable asset, and is why Google is willing to go through this.
