People constantly using this argument, but don't see how short sighted this is.

Celestia is not just a universal light switch, just like Luna's all existence is not devoted to rising moon.



Don't forget that in world of eternal darkness there is no need for Luna's talent. Moon is always on the sky. There is no need to raise him.



Remember that in episode "To Change a Changeling" ,Pharynx have similar dilemma.