Hate to be that guy, but... didn't it get mentioned on here a while back that the previous story idea was that neither side canonically ends up winning? There was some talking with Kurt Kuhlmann, one of Bethesda's lore guys;
@HTTP Error 404 brought it up
here.
Apparently, the original story beat was that the Thalmor ultimately won the second war regardless of whichever side won; the elves were going to officially take center stage as the ultimate villains of the setting, and ES6 was going to be all about going after them. Hence, why there was never a clear winner in Skyrim's story; both sides (were supposed to) end up losing in the end, regardless.