The theory of multiple universes was not only abandoned, but outpostulated long before time-travel ever began. The fact is, it doesn't really solve the grandfather paradox by entering a multiple universe, for the simple reason because it means that someone can't enter your universe's history and kill your grandfather, whether it's another you or someone else. Either way, you have "restricted" free will. The fact is, such other universes are unlikely to exist in the first place.