I've SEO'ed for quite a range of clients on behalf of an agency since I got into this game two years ago.
Generally, the clients who became millionaires generate 60-70% of their traffic away from Google, and SEO represents one of several revenue streams.
The clients who are losing money came to me because 'they want to get on the interweb' and 'get some of that google money'.
Risk aside, why wouldn't you develop other revenue streams? Even if you could guarantee and secure a high ranking in Google, those other revenue streams still represent good money, surely?