This Wall Street Journal article, How Search-Engine Rules Cause Sites to Go Missing, provides several examples of why relying on search engine driven traffic to your site as a primary strategy brings along some risks. Your business is subject to significant impact from relatively minor adjustments to the search engine algorithms and policies.
That said, the main example in the article is of a news web site that wants to change its domain name from a .net to a .com for branding reasons (after paying $1 million for the .com address):
Such a simple change, Mr. Skrenta has discovered, could have disastrous short-term results. About 50% of visits to his news site come through a search engine — and about 90% of the time, that is Google. Some companies say their sites have disappeared from top search results for weeks or months after making address switches, due to quirky rules Google and other search engines have adopted. So the same user who typed “Anna Nicole Smith news” into Google last week and saw Topix.net as a top result might not see it at all after the change to Topix.com.
Even if traffic to Topix, which gets about 10 million visitors a month, dropped just 10%, that would essentially be a 10% loss in ad revenue, Mr. Skrenta says. “Because of this little mechanical issue, it could be a catastrophe for us,” he says.
Since Google ascribes credibility to results on domains that it trusts, changing your domain name can have significant impact, as topix is discovering.
Any business model should be flexible enough to not be overly dependent on one source of business. For most organizations, search engine placement should be an important but not overarching strategy for the company.