In the past year, several tech companies that once seem inviolable have fractured badly—Research in Motion and Nokia most notably. Yahoo, which yesterday announced a surprising and energetic choice of Marissa Mayer as its new CEO, is not in such dire straits, but only just. Its revenues have been on a multiyear secular decline, occasionally flowing, mostly ebbing; it lost the battle with Google as a search engine; and its content, while stellar, may be unable to support its current structure and identity. Mayer is a bold hire, and visions of another turnaround, that of Apple in the late 1990s, must surely be a dream. The question will be whether that dream has any chance of becoming real.