If Xal isn't Old God 5, then I think #5 is in a state similar to how Elun'ahir was in DF- it's "another one of those things" (world trees/old gods) in a place we dont know (Khaz Algar/?????).
Probably felt their hands were tied to explain how Brann knows MOTHER for 8.3 to happen.
Unsure why they did the Zandalar intro but people getting mad about it are silly. It's dumb to change, but it still happened.