I find that I use google less than a couple of years ago. I think it is because the quality of destination sites has gone up, so I go directly to a handful of sites most of the time (admittedly gmail is one, but wikipedia, techmeme, nytimes, here, and so on). It's not that there's anything wrong with google, I just find myself searching less.
yahoo could never surrender all ads to google - the govt would never allow a cartel to control over 90% of web ads
most advertisers also are not interested in an ad cartel, which is why many continue to spend on yahoo and other ad networks