How will Stannis become the 1000th LC if the Wall falls? I mean, what would there be left to man or defend? Also, although clearly the show has diverged significantly, if Stannis takes the black as penance for sacrificing Shireen why would the show throw that possibility away by having him die at Winterfell? If your formulation were true, that seems like the sort of significant development they’d want to at least try to preserve.