It happens a fair amount. Here's just a quick list of players associated with the Sox that fell apart during their "prime years" - Rowand, Engel, Dye (had a resurgence), Dunn, Crede, Freddy Garcia, Jose Contreras (also had a resurgence). You can even make the argument for Frank, but Frank was still so good that he just transitioned into a pure power hitter after he lost all his bat speed.