So are you suggesting another split? I guess it would make sense if Link, Zelda, and Groose returned to their original present after defeating Demise in the past. That leaves their present unaltered, and the past with Demise dead is the new timeline. But that wouldn't explain the Master Sword...
That two Zeldas theory can be solved with Demise being "divided". When Link defeated him in the past, he wasn't completely destroyed, so Zelda didn't wake up in the past.
Okay, so I think everyone that has beaten this game can agree that the ending makes no sense. How can the game happen if at the end you destroy Demise in the past? It's a grandfather paradox too big to ignore. You could say that the timeline is split when Link goes back to the past to rescue...