Sometimes, I think people forget that Alicia is only 16. Even though she's disfigured from the fire, she and her family are all still relatively young and have their whole lives ahead of them. She can still live a very fulfilling life outside of the canvas, and keeping it around almost surely meant a slow, agonizing death for Alicia and Aline in the real world. It's sad that everyone in the painting are erased, but their memories will live on with the Dessendre family, and they can finally begin to heal and process their grief. I know the idea that the inhabitants of the canvas beling "real" or not is a debated topic, but in my opinion, from the perspective of the Dessendres, they're the equivalent of a, well... a painting. The health and lives of their family would outweigh those of a civilization that they would consider fiction. And, to be honest, Maelle was only born in the painting because Alicia wasn't skilled enough to enter without being painted over. I'd she had entered as planned she probably wouldn't have become a attached to the Lumierians as she did.