Right, but the argument goes that the only reason Google pays Mozilla so much is to prop up the competition. Google may have calculated that it would be preferable to pay the hundreds of millions, rather than have Firefox die just leaving Chrome/Safari.
Thus, it's not really a sustainable business, but rather being the lucky benefactor of some scheme. Their value is only in being 'the other guy', not in actually creating a valuable product for consumers. I wouldn't call that a sustainable business.