G'raha Tia didn't "erase a whole timeline because he misses his friends". He was trying to save the world, by going back in time and getting the person most likely to be able to save the First, and in the process, stop the 8th Umbral Calamity from happening. Were you reading the story at all?
Also, he didn't erase that timeline, it's still going on somewhere, he's just not there anymore. We actually have a short story about that.
