Again I say, as far as I am aware you do not pick your covenant until after you have quested for all of them and interacted with all of them and then you pick right at the end.
So, if your title said:
Imagine if once you got to end game in BFA you had to pick a covenant which would determine what corruptions, essences and azerite traits you were able to get
Well, then I think this discussions would have some silly ramifications.
Because, wow, it would be a whole lot easier to parse all the crap at the end game of BFA.