I think this would work for a short while but not long term.Big companies would also realize that Google is google and no search engines in the world could drive a huge traffic but Google.But in the process, online marketers like us would still benefit Afterwards, I bet big companies would still go back to Google.
My guess is Yahoo will require a contract that won't allow them to go back any time in the near future . These companies have a lot to consider, my guess is that they must be offered an unbelievable amount of money otherwise they would not even consider it.