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.