To some extent, the various scenarios that make up Margaret Atwood's "Happy Endings" are less important than how they end. Whether the variations on John and Mary's relationship are happy or sad, romantic or fraught with mutual animosity, they all end in the exact same way: in death.
For John and Mary, as for the other characters in the scenarios and ourselves, mortality is inescapable. Attwood is therefore all too aware of the fact that literary endings, the traditional conventions of works of literature, are, at best, provisional. They cannot have the last word; only death in the real world outside the text can do that.
As such, one could argue that Atwood is making a statement regarding the inherent artificiality of the literary text. Whatever John and Mary do or don't do, in the overall scheme of things, it doesn't much matter: first of all, because they are completely fictitious characters, and secondly, because even within the confines of the text, they must still die anyway.
In the final analysis, it doesn't much matter what John, Mary, or any of the other characters do, because they will all eventually die. What does matter, however, is how they do the things they do and why they do them, and it is here that death can play a vital role in providing us with the answers.