Okay everyone says there is always the possibility for something to come out of the blue and knock of the industry leader. Hey, Google did it back in the day.
But the relevancy factor is at the stage where it is going to rely on some type of personalization to make that next move. Human nature is such that this hurdle may be the one that stops someone from toppling Google from the top spot. Though the growth of China and other population exploding nations could be some type of impact factor in the search future.
I was talking to someone from Ask the other day and was trying to convince them to switch to a CPA (cost per acquisition) model. True it would involve reshaping their methods.... have a team that tests the CPA offers - counter offers etc. There would also be a need for a third party processing system so that the search engine was not losing the traffic when sites try and hijack the traffic, but ultimately it would open the market to many people who don't fully know how to work search.
Another way to steal a little thunder from Google - and listen Microsoft and Ask - is to offer agency commissions. They are available in Europe and Asia - give the agencies the ability to expense the process out better and they will push and convince those Fortune 500 companies that think search is a mionor play.