Magic power shouldn't deteriorate. Strength should deteriorate if a person just sits around playing video games however put 5 years in the context of Thor's 1,500 year life; and take into account his Asgardian physiology; the effect shouldn't be THAT much. Thor put on weight so he can't shoot lightning? That makes no sense. If Superman sat around for five years, would his heat vision not work anymore? Otherwise, if you say physical deterioration has an effect on magical power then Odin would be really weak as he clearly doesn't have the same physical attributes as Thor.