You may be right that thinning during maturation could serve a purpose, but if so you have the effect backwards: thinner myelin should mean higher capacitance (C ~ A/d) and slower conduction velocity. One would expect, if anything, that the myelin would need to get thicker as the limb lengthens. Perhaps nerve fiber diameter also increases with maturation---which also results in faster conduction velocity---and the myelin sheath thins to compensate.