Terry Semel alone cannot be held responsible for Yahoo's misfortunes. The problem has its root in Yahoo's line extension; the company has too many products (often competing against each other) irrelevant to its core business. Each time Yahoo ventures outside its core business, it sets itself open to a confrontation by a more focused competitor.
For instance, why did Yahoo 360 do terrible while other social networks gained market share? Because other social networks were intensely focused on their market, Yahoo wasn't.
For instance, why did Yahoo 360 do terrible while other social networks gained market share? Because other social networks were intensely focused on their market, Yahoo wasn't.