@tusooa There is so much interesting background story possible to develop around a magic mirror. I like to think about it like a Djinn who was imprisoned into a mirror and not into a magic lamp. But true: maybe breaking the mirror would release the Djinn, an unwanted outcome.
That reminds me of the character Gaunter O'Dimm on the Witcher 3 game (Heart of Stone DLC), one of the best written character I saw in a video game who is named 'master mirror'.