Nepy alluded to the reasons why we don't rewind permanents:
Because permanents have timers after which they are meant to be removed from play, and without such timers the permanent would become impossible to balance. Those permanents include:
Sundial, Dimensional Shield, Bonewall, Shard of Sacrifice.
Because we don't want to make draw-lock a competitive deck strategy. Adding more would decrease the impact of individual cards such as:
Reverse Time, Eternity, Nightmare.
