Well, everything that has a beginning, has an ending. Whether it has a happy ending or not is a different matter.
"Goodbye" is an important part of every end. We don't always hear this "goodbye" word, but somehow we can always feel this "goodbye". Always. It's like this bloody "goodbye" has a kinda ability to move through space and time.
Unlike (Adele's) "hello", "goodbye" isn't complicated message. It's straightforward.
And he made it quite clear.