sometimes the best ending for a character who has suffered and grieved and loved and lost and hurt is for them to be okay. for them to prove that you, too, can be okay, despite it all.
Ranboo was never meant to be of this world. His home, his family, was not of this world. He was created out of love.
Two entities gave him one of their lives, allowing him to be created and to live in this place. After adolescence they placed him here, hoping he would be safe.