It's also important to note that the $1B they pay Apple isn't insignificant either:
http://bgr.com/2013/02/11/google-apple-ios-default-search-1-...
some day Mozilla may pull the plug and switch to some other SE i.e. Bing or DDG
http://en.wikipedia.org/wiki/File:Usage_share_of_web_browser... (same marketshare now as in 2009.)
If it wasn't google paying, it would be microsoft, with bing.
It would be a huge loss for Google currently. Don't doubt for a second that if Firefox marketshare falls under 5% Google will stop paying anything.
http://www.zdnet.com/opera-google-extend-search-deal-for-two...
Opera makes about $15M per quarter in revenue from its desktop browser:
http://www.favbrowser.com/opera-software-q2-2012-financial-r...
Even Opera's marketshare is significant.
But just imagine, for a second, that 20% of the internet just gets to use bing instead of google.com because it became the default overnight. Outch.