User blog comment:Saratje/The Great Paarthurnax Conspiracy/@comment-3470121-20120417020753/@comment-1992639-20120419200301

As said, the grandfather paradox. The scroll allowed him to come back, before the scroll was opened. But if somehow the Dragonborn would die and the scroll would NOT be opened anymore, then his return would become undone. Best to google the grandfather-paradox for that. :)