I'm gonna say you haven't thought of a few things. Such as: (btw a YouTuber said this and i totally agree with it) Celestia is a literal embodiment of the sun. And, i think you have heard of the theories, that in a few million years or so, the sun will explode, or something like that. So, therefore, her sending Twilight or someone else to fight, she is protecting Equestria. You noticed how she doesn't always fight back? Maybe that's why. Same thing with Luna, but slightly, just slightly, less.