Is that an official Canonical position? Were you in on the discussion leading up to the switch back? A version of What you just said here would seem to be most appropriate as a clarification follow up to Rick's announcement. Since he didn't mention the revenue deal in the new posting.. but the revenue deal was the only rationale given in the original announcement.
Rick does however mention "unpredictable" circumstances? Are you implying that Canonical wasn't able to predict that current users who show a preference for Google? What's unpredictable about user opinion in this case? It should have been well known that Yahoo! provided less relevant for Ubuntu related search results than Google...that was something easily testable before Yahoo! was made the default. Something users tested for themselves. Did Canonical do that sort of testing internally prior to switching everyone to Yahoo!?
Now if there was only a way to estimate how many Ubuntu users are going to be using Yahoo! when its not the default...we could affirm whether that revenue deal is really still relevant or not.